Integrated power generation and carbon capture using fuel cells

ABSTRACT

Systems and methods are provided for capturing CO 2  from a combustion source using molten carbonate fuel cells (MCFCs). At least a portion of the anode exhaust can be recycled for use as a fuel for the combustion source. Optionally, a second portion of the anode exhaust can be recycled for use as part of an anode input stream. This can allow for a reduction in the amount of fuel cell area required for separating CO 2  from the combustion source exhaust and/or modifications in how the fuel cells can be operated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Application Ser. Nos.61/788,628, 61/787,587, 61/787,697, and 61/787,879, each filed on Mar.15, 2013; which are each incorporated by reference herein in theirentirety, as well as the three U.S. non-provisional applications filedon even date herewith and also claiming priority to the four provisionalapplications enumerated above, each of which non-provisionalapplications also being incorporated by reference herein in theirentirety.

FIELD OF THE INVENTION

In various aspects, the invention is related to low emission powerproduction with separation and/or capture of resulting emissions viaintegration of molten carbonate fuel cells with a combustion powersource.

BACKGROUND OF THE INVENTION

Capture of gases emitted from power plants is an area of increasinginterest. Power plants based on the combustion of fossil fuels (such aspetroleum, natural gas, or coal) generate carbon dioxide as a by-productof the reaction. Historically this carbon dioxide has been released intothe atmosphere after combustion. However, it is becoming increasinglydesirable to identify ways to find alternative uses for the carbondioxide generated during combustion.

One option for managing the carbon dioxide generated from a combustionreaction is to use a capture process to separate the CO₂ from the othergases in the combustion exhaust. An example of a traditional method forcapturing carbon is passing the exhaust stream through an aminescrubber. While an amine scrubber can be effective for separating CO₂from an exhaust stream, there are several disadvantages. In particular,energy is required to operate the amine scrubber and/or modify thetemperature and pressure of the exhaust stream to be suitable forpassing through an amine scrubber. The energy required for CO₂separation reduces the overall efficiency of the power generationprocess.

In order to offset the power required for CO₂ capture, one option is touse a molten carbonate fuel cell to assist in CO₂ separation. The fuelcell reactions that cause transport of CO₂ from the cathode portion ofthe fuel cell to the anode portion of the fuel cell can also result ingeneration of electricity. However, conventional combinations of acombustion powered turbine or generator with fuel cells for carbonseparation have resulted in a net reduction in power generationefficiency per unit of fuel consumed.

An article in the Journal of Fuel Cell Science and Technology (G.Manzolini et. al., J. Fuel Cell Sci. and Tech., Vol. 9, February 2012)describes a power generation system that combines a combustion powergenerator with molten carbonate fuel cells. The combustion output fromthe combustion generator is used in part as the input for the cathode ofthe fuel cell. This input is supplemented with a recycled portion of theanode output after passing through the anode output through a cryogenicCO₂ separator.

An article by Desideri et al. (Intl. J. of Hydrogen Energy, Vol. 37,2012) describes a method for modeling the performance of a powergeneration system using a fuel cell for CO₂ separation. Recirculation ofanode exhaust to the anode inlet and the cathode exhaust to the cathodeinlet are used to improve the performance of the fuel cell. Based on themodel and configuration shown in the article, increasing the CO₂utilization within the fuel cell is shown as being desirable forimproving separation of CO₂.

U.S. Pat. No. 7,396,603 describes an integrated fossil fuel power plantand fuel cell system with CO₂ emissions abatement. At least a portion ofthe anode output is recycled to the anode input after removal of aportion of CO₂ from the anode output.

SUMMARY OF THE INVENTION

In one aspect of the invention, a method for capturing carbon dioxidefrom a combustion source is provided. The method can include:introducing one or more fuel streams and an O₂-containing stream into acombustion zone; performing a combustion reaction in the combustion zoneto generate a combustion exhaust, the combustion exhaust comprising CO₂;processing at least a first portion of the combustion exhaust with afuel cell array of one or more molten carbonate fuel cells to form acathode exhaust stream from at least one cathode outlet of the fuel cellarray, the one or more fuel cells each having an anode and a cathode,the molten carbonate fuel cells being operatively connected to thecombustion exhaust through one or more cathode inlets of fuel cells inthe fuel cell array; reacting carbonate from the one or more fuel cellcathodes with hydrogen within the one or more fuel cell anodes toproduce electricity, an anode exhaust stream from at least one anodeoutlet of the fuel cell array comprising CO₂ and H₂: separating CO₂ fromthe anode exhaust stream in one or more separation stages to form aCO₂-depleted anode exhaust stream; and passing at least a first portionof the CO₂-depleted anode exhaust stream to the combustion zone.Optionally, the method can further comprise recycling at least a secondportion of the CO₂-depleted anode exhaust stream to the anode.

In another aspect of the invention, a system for power generation isprovided. The system can include: a combustion turbine including acompressor, the compressor receiving an oxidant input and being in fluidcommunication with a combustion zone, the combustion zone furtherreceiving a first fuel input and a second fuel input, the combustionzone being in fluid communication with an expander having an exhaustoutput; an exhaust gas recirculation system providing fluidcommunication between a first portion of the expander exhaust output andthe combustion zone: a fuel cell array having at least one cathodeinput, at least one cathode output, at least one anode input, and atleast one anode output, a second portion of the expander exhaust outputbeing in fluid communication with the at least one cathode input; and ananode recycle loop comprising one or more carbon dioxide separationstages, a first portion of an anode recycle loop output being providedto the combustion zone as at least a portion of the second fuel input.Optionally, a second portion of the anode recycle loop output isprovided to the anode input.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an example of a combined cycle system forgenerating electricity based on combustion of a carbon-based fuel.

FIG. 2 schematically shows an example of the operation of a moltencarbonate fuel cell.

FIG. 3 shows an example of the relation between anode fuel utilizationand voltage for a molten carbonate fuel cell.

FIG. 4 schematically shows an example of a configuration for an anoderecycle loop.

FIG. 5 shows an example of the relation between CO₂ utilization,voltage, and power for a molten carbonate fuel cell.

FIG. 6 schematically shows another example of a combined cycle systemfor generating electricity based on combustion of a carbon-based fuel.

FIGS. 7 and 8 show results from simulations of various configurations ofa power generation system including a combustion-powered turbine and amolten carbonate fuel cell for carbon dioxide separation.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In various aspects, systems and methods are provided for capturing CO₂from a combustion source using molten carbonate fuel cells (MCFCs). Thesystems and methods can address one or more problems related to carboncapture from combustion exhaust stream and/or performing carbon captureusing molten carbonate fuel cells.

One difficulty with using molten carbonate fuel cells for separation ofCO₂ from an exhaust stream can include the large area of fuel cellstypically required for handling the exhaust from a commercial scaleturbine or other power/heat generator. Accommodating a commercial scaleexhaust flow using molten carbonate fuel cells can typically involveusing a plurality of fuel cells, rather than constructing a single fuelcell of sufficient area. In order to deliver the exhaust stream to thisplurality of fuel cells, additional connections can be required in orderto divide the exhaust between the various fuel cells. Thus, reducing thefuel cell area required to capture a desired amount of carbon dioxidecan provide a corresponding decrease in the number and/or complexity offlow connections required.

In some aspects of the invention, the area of fuel cells required forprocessing a CO₂-containing exhaust stream can be reduced or minimizedby recycling at least a portion of the anode exhaust stream back to theanode inlet. Additionally or alternately, the fuel cells can be operatedat lower fuel utilization. An exhaust stream can be passed into thecathode(s) of molten carbonate fuel cells. During operation of the fuelcell, the anode exhaust can be passed through one or more separationstages. This can include separation stages for removal of H₂O and/orCO₂. At least a portion of the remaining anode exhaust can then berecycled to the anode input. In one preferred embodiment, any recycle ofthe anode exhaust directly to the cathode can be avoided. By recyclingat least some portion the anode exhaust to the anode inlet, at leastsome of the fuel not used on the first pass through the anode can beutilized in a subsequent pass.

In addition to or as an alternative to recycling the anode exhaust tothe anode inlet, at least a portion of the hydrogen in the anode exhaustcan be recycled to the combustion zone for a turbine or othercombustion-powered generator/heat source. It is noted that any hydrogengenerated via reforming as part of the anode loop can represent a fuelwhere the CO₂ has already been “captured” by transfer to the anode loop.This can reduce the amount of CO₂ needing to be transferred from thecathode side to the anode side of the fuel cell, and therefore can leadto a reduced fuel cell area.

An additional or alternative feature that can contribute to a reducedfuel cell area can be reducing and/or minimizing the amount of energyrequired for processes not directly involved in power generation. Forexample, the anode reaction in a molten carbonate fuel cell can combineH₂ with CO₃ ²⁻ ions transported across the electrolyte between cathodeand anode to form H₂O and CO₂. Although the anode reaction environmentcan facilitate some reforming of a fuel such as CH₄ to form H₂, some H₂can advantageously be present in a fuel in order to maintain desirablereaction rates in the anode. As a result, prior to entering the anodeitself, fuels (such as natural gas/methane) are conventionally at leastpartially reformed prior to entering the anode. The reforming stageprior to the anode for a fuel cell can require additional heat in orderto maintain a suitable temperature for reforming.

In some aspects of the invention, recycling at least a portion of theanode exhaust to the anode inlet can allow for a reduced amount ofreforming and/or elimination of the reforming stage prior to the anodeinlet. Instead of reforming a fuel stream prior to entering the anode,the recycled anode exhaust can provide sufficient hydrogen for the fuelinput to the anode. This can allow the input stream for the anode to bepassed into the anode without passing through a separate pre-reformingstage. Operating the anode at a reduced level of hydrogen fuelutilization can further facilitate reducing and/or eliminating thepre-reforming stage by providing an anode exhaust with increasedhydrogen content. Increasing the hydrogen content can allow a portion ofthe anode exhaust to also be used as an input to the turbine combustionzone, while still having sufficient hydrogen in the feed to the anodeinlet so that pre-reforming can be reduced and/or eliminated.

Another challenge with using molten carbonate fuel cells can be due tothe relatively low CO₂ content of the exhaust of properly operated gasturbine. For example, a gas turbine powered by a low CO₂ content naturalgas fuel source can generate an exhaust, for example, with a CO₂ ofabout 4 vol %. If some type of exhaust gas recycle is used, this valuecan be raised, for example, to about 6 vol %. By contrast, a typicaldesired CO₂ content for the input to the cathode of a molten carbonatefuel cell can be about 10% or more. In some aspects of the invention,systems and methods are provided herein to allow for increased CO₂content in the exhaust gas while still efficiently operating the gasturbine or other combustion powered generator. In some aspects of theinvention, systems and methods are provided for improving and/oroptimizing the efficiency of carbon capture by the fuel cell whenoperated with a cathode exhaust having a low CO₂ content.

Still another challenge can include reducing or mitigating the loss ofefficiency in power generation caused by carbon capture. As noted above,conventional methods of carbon capture can result in a loss of netefficiency in power generation per unit of fuel consumed. In someaspects of the invention, systems and methods are provided for improvingthe overall power generation efficiency. Additionally or alternately, insome aspects of the invention, methods are provided for separating CO₂in a manner to reduce and/or minimize the energy required for generationof a commercially valuable CO₂ stream.

In most aspects of the invention, one or more of the above advantagescan be achieved, at least in part, by using molten carbonate fuel cellsin combination with a combined cycle power generation system, such as anatural gas fired combined cycle plant, where the flue gas and/or heatfrom combustion reaction(s) can also be used to power a steam turbine.More generally, the molten carbonate fuel cells can be used inconjunction with various types of power or heat generation systems, suchas boilers, combustors, catalytic oxidizers, and/or other types ofcombustion powered generators. In some aspects of the invention, atleast a portion of the anode exhaust from the MCFCs can be (afterseparation of CO₂) recycled to the input flow for the MCFC anode(s).Additionally or alternately, a portion of the anode exhaust from theMCFCs can be recycled to the input flow for the combustion reaction forpower generation. In aspects where the MCFCs can be operated withremaining (unreacted) H₂ in the anode exhaust, recycling a portion ofthe H₂ from the anode exhaust to the anode input can reduce the fuelneeded for operating the MCFCs. The portion of H₂ delivered to thecombustion reaction can advantageously modify and/or improve reactionconditions for the combustion reaction, leading to more efficient powergeneration. A water-gas shift reaction zone after the anode exhaust canoptionally be used to further increase the amount of H₂ present in theanode exhaust while also allowing conversion of CO into more easilyseparable CO₂.

In various aspects of the invention, an improved method for capturingCO₂ from a combustion source using a molten carbonate fuel cell can beprovided. This can include, for example, systems and methods for powergeneration using turbines (or other power or heat generation methodsbased on combustion, such as boilers, combustors, and/or catalyticoxidizers) while reducing and/or mitigating emissions during powergeneration. This can optionally be achieved, at least in part, by usinga combined cycle power generation system, where the flue gas and/or heatfrom combustion reaction(s) can also be used to power a steam turbine.This can additionally or alternately be achieved, at least in part, byusing one or more molten carbonate fuel cells (MCFCs) as both a carboncapture device as well as an additional source of electrical power. Insome aspects of the invention, the MCFCs can be operated under low fuelutilization conditions that can allow for improved carbon capture in thefuel cell while also reducing and/or minimizing the amount of fuel lostor wasted. Additionally or alternately, the MCFCs can be operated toreduce and/or minimize the total number and/or volume of MCFCs requiredto reduce the CO₂ content of a combustion flue gas stream to a desiredlevel, for example, 1.5 vol % or less or 1.0 vol % or less. Such aspectscan be enabled, at least in part, by recycling the exhaust from theanode back to the inlet of the anode, with removal of at least a portionof the CO₂ in the anode exhaust prior to returning the anode exhaust tothe anode inlet. Such removal of CO₂ from the anode exhaust can beachieved, for example, using a cryogenic CO₂ separator. In some optionalaspects of the invention, the recycle of anode exhaust to the anodeinlet can be performed so that no pathway is provided for the anodeexhaust to be recycled directly to the cathode inlet. By avoidingrecycle of anode exhaust directly to the cathode inlet, any CO₂transported to the anode recycle loop via the MCFCs can remain in theanode recycle loop until the CO₂ is separated out from the other gasesin the loop.

Molten carbonate fuel cells are conventionally used in a standalone modeto generate electricity. In a standalone mode, an input stream of fuel,such as methane, can be passed into the anode side of a molten carbonatefuel cell. The methane can be reformed (either externally or internally)to form H₂ and other gases. The H₂ can then be reacted with carbonateions that have crossed the electrolyte from the cathode in the fuel cellto form CO₂ and H₂O. For the reactions in the anode of the fuel cell,the rate of fuel utilization is typically about 70% or 75%, or evenhigher. In a conventional configuration, the remaining fuel in the anodeexhaust can be oxidized (burned) to generate heat for maintaining thetemperature of the fuel cell and/or external reformer, in view of theendothermic nature of the reforming reaction. Air and/or another oxygensource can be added during this oxidation to allow for more completecombustion. The anode exhaust (after oxidation) can then be passed intothe cathode. In this manner, a single fuel stream entering the anode canbe used to provide all of the energy and nearly all of the reactants forboth anode and cathode. This configuration can also allow all of thefuel entering the anode to be consumed while only requiring ˜70% or ˜75%or slightly more fuel utilization in the anode.

In the above standalone method, typical of conventional systems, thegoal of operating a molten carbonate fuel cell is generally toefficiently generate electric power based on an input fuel stream. Bycontrast, a molten carbonate fuel cell integrated with a combustionpowered turbine, engine, or other generator can be used to provide adifferent utility. Although power generation by the fuel cell is stilldesirable, the fuel cell can be operated, for instance, to improveand/or maximize the amount of CO₂ captured from an exhaust stream for agiven volume of fuel cells. This can allow for improved CO₂ capturewhile still generating power from the fuel cell. Additionally, in someaspects of the invention, the exhaust from the anode(s) of the fuelcell(s) can still contain excess hydrogen. This excess hydrogen canadvantageously be used as a fuel for the combustion reaction for theturbine, thus allowing for improved efficiency for the turbine.

FIG. 1 provides a schematic overview for the concept of some aspects ofthe invention. FIG. 1 is provided to aid in understanding of the generalconcept, so additional feeds, processes, and or configurations can beincorporated into FIG. 1 without departing 11 from the spirit of theoverall concept. In the overview example shown in FIG. 1, a natural gasturbine 110 (or another combustion-powered turbine) can be used togenerate electric power based on combustion of a fuel 112. For thenatural gas turbine 110 shown in FIG. 1, this can include compressing anair stream or other gas phase stream 111 to form a compressed gas stream113. The compressed gas stream 113 can then be introduced into acombustion zone 115 along with fuel 112. Optionally, a stream 185,including a portion of the fuel (hydrogen) present in the exhaust fromanode 130, can additionally or alternately be introduced into thecombustion zone 115. This additional hydrogen can allow the combustionreaction to be operated under enhanced conditions. The resulting hotflue or exhaust gas 117 can then be passed into the expander portion ofturbine 110 to generate electrical power.

After expansion (and optional clean up and/or other processing steps),the expanded flue gas can be passed into the cathode portion 120 of amolten carbonate fuel cell. The flue gas can include sufficient oxygenfor the reaction at the cathode, or additional oxygen can be provided ifnecessary. To facilitate the fuel cell reaction, fuel 132 can be passedinto the anode portion 130 of the fuel cell, along with at least aportion of the anode exhaust 135. Prior to being recycled, the anodeexhaust 135 can be passed through several additional processes. Oneadditional process can include or be a water-gas shift reaction process170. The water gas-shift reaction 170 can be used to react H₂O and COpresent in the anode exhaust 135 to form additional H₂ and CO₂. This canallow for improved removal of carbon from the anode exhaust 135, as CO₂can typically be more readily separated from the anode exhaust, ascompared to CO. The output 175 from the (optional) water-gas shiftprocess 170 can then be passed through a carbon dioxide separationsystem 140, such as a cryogenic carbon dioxide separator. This canremove at least a portion of CO₂ 147 from the anode exhaust, typicallyas well as a portion of the water 149 also. After removal of at least aportion of the CO₂ and water, the recycled anode exhaust can stillcontain some CO₂ and water, as well as unreacted fuel in the form of H₂and/or possibly a hydrocarbon such as methane. In certain embodiments ofthe invention, a portion 145 of the output from the CO₂ separationstage(s) can be recycled for use as an input stream to anode 130, whilea second portion can be used as input 185 to the combustion reaction115. Fuel 132 can represent a hydrogen-containing stream and/or a streamcontaining methane and/or another hydrocarbon that can be reformed(internally or externally) to form H₂.

The exhaust from the cathode portion 120 of the fuel cell can then bepassed into a heat recovery zone 150 so that heat from the cathodeexhaust can be recovered, e.g., to power a steam generator 160. Afterrecovering heat, the cathode exhaust can exit the system as an exhauststream 156. The exhaust stream 156 can exit to the environment, oroptional additional clean-up processes can be used, such as performingadditional CO₂ capture on stream 156, for example, using an aminescrubber.

Molten Carbonate Fuel Cell

In various aspects of the invention, a molten carbonate fuel cell (MCFC)can be used to facilitate separation of CO₂ from a CO₂-containing streamwhile also generating additional electrical power. The CO₂ separationcan be further enhanced by taking advantage of synergies with thecombustion-based power generator that can provide at least a portion ofthe input feed to the cathode portion of the fuel cell.

In this discussion, a fuel cell can correspond to a single cell, with ananode and a cathode separated by an electrolyte. The anode and cathodecan receive input gas flows to facilitate the respective anode andcathode reactions for transporting charge across the electrolyte andgenerating electricity. A fuel cell stack can represent a plurality ofcells in an integrated unit. Although a fuel cell stack can includemultiple fuel cells, the fuel cells can typically be connected inparallel and can function (approximately) as if they collectivelyrepresented a single fuel cell of a larger size. When an input flow isdelivered to the anode or cathode of a fuel cell stack, the fuel stackcan include flow channels for dividing the input flow between each ofthe cells in the stack and flow channels for combining the output flowsfrom the individual cells. In this discussion, a fuel cell array can beused to refer to a plurality of fuel cells (such as a plurality of fuelcell stacks) that are arranged in series, in parallel, or in any otherconvenient manner (e.g., in a combination of series and parallel). Afuel cell array can include one or more stages of fuel cells and/or fuelcell stacks, where the anode/cathode output from a first stage may serveas the anode/cathode output for a second stage. It is noted that theanodes in a fuel cell stage do not have to be connected in the same wayas the cathodes in a stage. For convenience, the input to the firstanode stage of a fuel cell array may be referred to as the anode inputfor the array, and the input to the first cathode stage of the fuel cellarray may be referred to as the cathode input to the array. Similarly,the output from the final anode/cathode stage may be referred to as theanode/cathode output from the array.

It should be understood that reference to use of a fuel cell hereintypically denotes a “fuel cell stack” composed of individual fuel cells,and more generally refers to use of one or more fuel cell stacks influid communication. Individual fuel cell elements (plates) cantypically be “stacked” together in a rectangular array called a “fuelcell stack”. This fuel cell stack can typically take a feed stream anddistribute reactants among all of the individual fuel cell elements andcan then collect the products from each of these elements. When viewedas a unit, the fuel cell stack in operation can be taken as a whole eventhough composed of many (often tens or hundreds) of individual fuelcells. These individual fuel cells can typically have similar voltages(as the reactant and product concentrations are similar), and the totalpower output can result from the summation of all of the electricalcurrents in all of the cells. Stacks can also be arranged in aseries/parallel arrangement to result in high voltages. If asufficiently large volume fuel cell stack is available to process agiven exhaust flow, the systems and methods described herein can be usedwith a single molten carbonate fuel cell stack. In other aspects of theinvention, a plurality of fuel cell stacks may be desirable or neededfor a variety of reasons.

For the purposes of this invention, unless otherwise specified, the term“fuel cell” should be understood to also refer to and/or is defined asincluding a reference to a fuel cell stack composed of set of one ormore individual fuel cells for which there is a single input and output,as that is the manner in which fuel cells are typically employed inpractice. Similarly, the term fuel cells (plural), unless otherwisespecified, should be understood to also refer to and/or is defined asincluding a plurality of separate fuel cell stacks. In other words, allreferences within this document, unless specifically noted, can referinterchangeably to the operation of a fuel cell stack as a “fuel cell”.For example, the volume of exhaust generated by a commercial scalecombustion generator may be too large for processing by a fuel cell(i.e., a single stack) of conventional size. In order to process thefull exhaust, a plurality of fuel cells (i.e., two or more separate fuelcells or fuel cell stacks) can be arranged in parallel, so that eachfuel cell processes (roughly) an equal portion of the combustionexhaust. Although multiple fuel cells can be used, each fuel cell can beoperated in a generally similar manner.

One way of characterizing the operation of a fuel cell can be tocharacterize the “utilization” of various inputs received by the fuelcell. For example, one common method for characterizing the operation ofa fuel cell can be to specify the (anode) fuel utilization for the fuelcell.

Fuel cell fuel utilization as used herein can be computed using the flowrates and Lower Heating Value (LHV) of the fuel components entering andleaving the fuel cell anode. Lower heating value is defined as theenthalpy of combustion of a fuel component to vapor phase, fullyoxidized products (i.e., vapor phase CO₂ and H₂O product). As such, fuelutilization (Uf) can be computed as Ur=(LHV(in)−LHV(out))/LHV(in), whereLHV(in) and LHV(out) refer to the LHV of the fuel components (such asH₂. CH₄, and/or CO) in the anode inlet and outlet streams or flows,respectively. In this definition, the LHV of a stream or flow may becomputed as a sum of values for each fuel component in the input and/oroutput stream. The contribution of each fuel component to the sum cancorrespond to the fuel component's flow rate (e.g., mol/hr) multipliedby the fuel component's LHV (e.g., joules/mol). It is noted thatcomponents in the anode input flow that do not participate in acombustion reaction to form H₂O and/or CO₂ are not typically considered“fuel components”.

It is noted that, for the special case where the only fuel in the anodeinput flow is H₂, the only reaction involving a fuel component that cantake place in the anode represents the conversion of H₂ into H₂O. Inthis special case, the fuel utilization simplifies to (H₂-rate-in minusH₂-rate-out)/H₂-rate-in. In such a case. H₂ would be the only fuelcomponent, and so the H₂ LHV would cancel out of the equation. In themore general case, the anode feed may contain, for example, CH₄, H₂, andCO in various amounts. Because these species can typically be present indifferent amounts in the anode outlet, the summation as described abovecan be needed to determine the fuel utilization.

In addition to fuel utilization, the utilization for other reactants inthe fuel cell can be characterized. For example, the operation of a fuelcell can additionally or alternately be characterized with regard to“CO₂ utilization” and/or “oxidant” utilization. The values for CO₂utilization and/or oxidant utilization can be specified in a similarmanner. For CO₂ utilization, the simplified calculation of (CO₂-ratc-inminus CO₂-rate-out)/CO₂-rate-in can be used if CO₂ is the only fuelcomponent present in the input stream or flow to the cathode, with theonly reaction thus being the formation of CO₃ ²⁻. Similarly, for oxidantutilization, the simplified version can be used if O₂ is the onlyoxidant present in the input stream or flow to the cathode, with theonly reaction thus being the formation of CO₃ ²⁻.

Another reason for using a plurality of fuel cells can be to allow forefficient fuel cell operation while reducing the CO₂ content of thecombustion exhaust to a desired level. Rather than operating a fuel cellto have a high (or optimal) rate of CO₂ utilization, two (or more) fuelcells can be operated at lower fuel utilization rate(s) while reducingthe combustion to a desired level.

FIG. 2 shows a schematic example of the operation of an MCFC forgeneration of electrical power. In FIG. 2, the anode portion of the fuelcell can receive fuel and steam (H₂O) as inputs, with outputs of water,CO₂, and optionally excess H₂, CH₄ (or other hydrocarbons), and/or CO.The cathode portion of the fuel cell can receive CO₂ and some oxidant(e.g., air/O₂) as inputs, with an output corresponding to a reducedamount of CO₂ in O₂-depleted oxidant (air). Within the fuel cell, CO₃ ²⁻ions formed in the cathode side can be transported across the cathode(membrane), through the electrolyte in the fuel cell, and across theanode (membrane) to provide the carbonate ions needed for the reactionsoccurring at the anode.

In a molten carbonate fuel cell such as the example fuel cell shown inFIG. 2, there are three basic reactions that can occur. The firstreaction can be optional, and can be reduced or eliminated if sufficientH₂ is provided directly to the anode.

<anode> CH₄+2H₂O=>4H₂+CO₂

<anode> 4H₂+4CO₃ ²⁻=>4H₂O+4CO₂+8e ⁻

<cathode> 2O₂+4CO₂+8e ⁻=>4CO₃ ²⁻

Reaction (1) represents a hydrocarbon reforming reaction to generate H₂for use in the anode of the fuel cell. Reaction (1) can occur externalto the fuel cell, and/or the reforming can be performed internal to thefuel cell. Reaction (1) can be optional, as the primary purpose ofreaction (1) is to generate H₂. The CO₂ generated by reaction (1) doesnot generally undergo further reaction within the fuel cell, and, to afirst approximation, thus does not significantly impact reaction (2).

Reactions (2) and (3), at the anode and cathode respectively, representthe reactions that can result in electrical power generation within thefuel cell. Reaction (2) combines H₂, optionally generated by reaction(1), with carbonate ions to form H₂O, CO₂, and electrons. Reaction (3)combines O₂, CO₂, and electrons to form carbonate ions. The carbonateions generated by reaction (3) can be transported across the electrolyteof the fuel cell to provide the carbonate ions needed for reaction (2).In combination with the transport of carbonate ions across theelectrolyte, a closed current loop can then be formed by providing anelectrical connection between the anode and cathode.

During conventional operation of a fuel cell, such as standaloneoperation, the goal of operating the fuel cell can be to generateelectrical power while efficiently using the “fuel” provided to thecell. The “fuel” can correspond to either hydrogen (H₂), a gas streamcomprising hydrogen, and/or a gas stream comprising a substance that canbe reformed to provide hydrogen (such as methane, another alkane orhydrocarbon, and/or one or more other types of compounds containingcarbon and hydrogen that, upon reaction, can provide hydrogen). Thesereforming reactions are typically endothermic and thus usually consumesome heat energy in the production of hydrogen. Carbon sources that canprovide CO directly and/or upon reaction can also be utilized, astypically the water gas shift reaction (CO +H₂O═H₂+CO₂) can occur in thepresence of the fuel cell anode catalyst surface. This can allow forproduction of hydrogen from a CO source. For such conventionaloperation, one potential goal of operating the fuel cell can be toconsume all of the fuel provided to the cell, while maintaining adesirable output voltage for the fuel cell, which can be traditionallyaccomplished by operating the fuel cell anode at a fuel utilization ofabout 70% to about 75%, followed by oxidizing (such as burning) theremaining fuel to generate heat to maintain the temperature of the fuelcell.

By contrast, in various embodiments, the goal of operating the fuel cellcan be to separate CO₂ from the output stream of a combustion reaction,in addition to allowing generation of electric power. In suchembodiments, the combustion reaction(s) can be used to power one or moregenerators or turbines, which provide the majority of the powergenerated by the combined generator/fuel cell system. Rather thanoperating the fuel cell to optimize power generation by the fuel cell,the system can instead be operated to improve the capture of carbondioxide from the combustion-powered generator while reducing and/orminimizing the number of fuels cells required for capturing the carbondioxide. Selecting an appropriate configuration for the input and outputflows of the fuel cell, as well as selecting appropriate operatingconditions for the fuel cell, can allow for a desirable combination ofoverall power generation efficiency and carbon capture. One aspect ofselecting appropriate operating conditions can correspond to selectingoperating conditions based on a factor other than fuel utilization. Interms of fuel utilization, the operating conditions can result in lowerfuel utilization than in a conventional fuel cell.

In various aspects of the invention where fuel cells are operated tohave a low fuel utilization, a molten carbonate fuel cell can beoperated to have a fuel utilization of about 65% or less, for example,about 60% or less, about 55% or less, about 50% or less, or about 45% orless. Additionally or alternately, a molten carbonate fuel cell can beoperated to have a fuel utilization of at least about 25%, for exampleat least about 30%, at least about 35%, or at least about 40%.

In some embodiments, the fuel cells in a fuel cell array can be arrangedso that only a single stage of fuel cells (such as fuel cell stacks) canbe present. In this type of embodiment, the anode fuel utilization forthe single stage can represent the anode fuel utilization for the array.Another option can be that a fuel cell array can contain multiple stagesof anodes and/or multiple stages of cathodes, with each anode stagehaving a fuel utilization within the same range, such as each anodestage having a fuel utilization within 10% of a specified value, forexample within 5% of a specified value. Still another option can be thateach anode stage can have a fuel utilization equal to a specified valueor lower than the specified value by less than an amount, such as havingeach anode stage be not greater than a specified value by 10% or less,for example, by 5% or less. As an illustrative example, a fuel cellarray with a plurality of anode stages can have each anode stage bewithin about 10% of 50% fuel utilization, which would correspond to eachanode stage having a fuel utilization between about 40% and about 60%.As another example, a fuel cell array with a plurality of stages canhave each anode stage be not greater than 60% anode fuel utilizationwith the maximum deviation being about 5% less, which would correspondto each anode stage having a fuel utilization between about 55% to about60%. In still another example, one or more stages of fuel cells in afuel cell array can be operated at a fuel utilization from about 30% toabout 50%, such as operating a plurality of fuel cell stages in thearray at a fuel utilization from about 30% to about 50%. More generally,any of the above types of ranges can be paired with any of the anodefuel utilization values specified herein.

Still another option can include specifying a fuel utilization for lessthan all of the anode stages. For example, in some aspects of theinvention where fuel cells/stacks are arranged at least partially in oneor more series arrangements, that anode fuel utilization can bespecified for the first anode stage in a series, the second anode stagein a series, the final anode stage in a series, or any other convenientanode stage in a series. As used herein, the “first” stage in a seriescorresponds to the stage (or set of stages, if the arrangement containsparallel stages as well) to which input is directly fed from the fuelsource(s), with later (“second”, “third”, “final”, etc.) stagesrepresenting the stages to which the output from one or more previousstages is fed, instead of directly from the respective fuel source(s).In situations where both output from previous stages and input directlyfrom the fuel source(s) are co-fed into a stage, there can be a “first”(set of) stage(s) and a “last” (set of) stage(s), but other stages(“second”, “third”, etc.) can be more tricky among which to establish anorder (e.g., in such cases, ordinal order can be determined byconcentration levels of one or more components in the composite inputfeed composition, such as CO₂ for instance, from highest concentration“first” to lowest concentration “last” with approximately similarcompositional distinctions representing the same ordinal level.

Yet another option can be to specify the anode fuel utilizationcorresponding to a particular cathode stage (again, where fuelcells/stacks are arranged at least partially in one or more seriesarrangements). As noted above, based on the direction of the flowswithin the anodes and cathodes, the first cathode stage may notcorrespond to (be across the same fuel cell membrane from) the firstanode stage. Thus, in some aspects of the invention, the anode fuelutilization can be specified for the first cathode stage in a series,the second cathode stage in a series, the final cathode stage in aseries, or any other convenient cathode stage in a series.

Yet still another option can be to specify an overall average of fuelutilization over all fuel cells in a fuel cell array. In variousaspects, the overall average of fuel utilization for a fuel cell arraycan be about 65% or less, for example, about 60% or less, about 55% orless, about 50% or less, or about 45% or less (additionally oralternately, the overall average fuel utilization for a fuel cell arraycan be at least about 25%, for example at least about 30%, at leastabout 35%, or at least about 40%). Such an average fuel utilization neednot necessarily constrain the fuel utilization in any single stage, solong as the array of fuel cells meets the desired fuel utilization.

In a molten carbonate fuel cell, the transport of carbonate ions acrossthe electrolyte in the fuel cell can provide a method for transportingCO₂ from a first flow path to a second flow path, where the transportmethod can allow transport from a lower concentration (the cathode) to ahigher concentration (the anode), which can thus facilitate capture ofCO₂. For embodiments where the input to the cathode can be primarilybased on the output gas from a combustion reaction for powering aturbine or another type of power generator, the CO₂ content of theoutput gas can tend to be relatively low in comparison to the totaloutput gas composition. For example, the CO₂ content of the output froma natural gas combustion turbine can be from about 3 vol % to about 6vol %, although higher CO₂ contents can be possible, e.g., for turbineconfigurations including exhaust gas recovery. Coal-fired power plantscan have higher CO₂ concentrations, such as up to about 15 vol % ormore. For such output flows, the majority of the gas in the output flowcan be nitrogen, especially if the source of oxidant for the combustionreaction is air or a primarily nitrogen-containing gas. Due to therelatively low concentration of CO₂, one of the challenges in separatingout the CO₂ from such streams can be related to performing acost-effective separation resulting in a relatively high purity CO₂output stream. An MCFC can be beneficial for performing this type ofseparation, as part of the selectivity of the fuel cell can be based onthe electrochemical reactions allowing the cell to generate electricalpower. For non-reactive species (such as N₂) that effectively do notparticipate in the electrochemical reactions within the fuel cell, therecan be an insignificant amount of reaction and transport from cathode toanode. By contrast, the potential (voltage) difference between thecathode and anode can provide a strong driving force for transport ofcarbonate ions across the fuel cell. As a result, the transport ofcarbonate ions in the molten carbonate fuel cell can allow CO₂ to betransported from the cathode (lower CO₂ concentration) to the anode(higher CO₂ concentration) with relatively high selectivity.

Operation of Anode Portion and Anode Recycle Loop

In various aspects of the invention, molten carbonate fuel cells can beoperated under conditions that allow for lower fuel utilization in theanode portion of the fuel cell. This is in contrast to conventionaloperation for fuel cells, where the fuel utilization is typicallyselected in order to allow all of the fuel delivered to the fuel cell tobe consumed as part of operation of the fuel cell. In conventionaloperation, all of the fuel is typically either consumed within the anodeof the fuel cell or burned to provide sensible heat for the feed streamsto the fuel cell.

FIG. 3 shows an example of the relationship between fuel utilization andoutput power for a fuel cell operating under conventional (stand-alone)conditions. The diagram shown in FIG. 3 shows two limiting cases foroperation of a fuel cell. One limiting case includes the limit ofoperating a fuel cell to consume an amount of fuel (such as H₂ ormethane reformed into H₂) that approaches 100% of the fuel delivered tothe fuel cell. From an efficiency standpoint, consumption of ˜100% ofthe fuel delivered to a fuel cell would be desirable, so as not to wastefuel during operation of the fuel cell. However, there are two potentialdrawbacks with operating a fuel cell to consume more than about 80% ofthe fuel delivered to the cell. First, as the amount of fuel consumedapproaches 100%, the voltage provided by the fuel cell can be sharplyreduced. In order to consume an amount of fuel approaching 100%, theconcentration of the fuel in the fuel cell (or at least near the anode)must almost by definition approach zero during at least part of theoperation of the fuel cell. Operating the anode of the fuel cell with afuel concentration approaching zero can result in a decreasingly lowdriving force for transporting carbonate across the electrolyte of thefuel cell. This can cause a corresponding drop in voltage, with thevoltage potentially also approaching zero in the true limiting case ofconsuming all fuel provided to the anode.

The second drawback is also related to relatively high fuel utilizationvalues (greater than about 80%) even if consumption does not approach100%. As shown in FIG. 3, at fuel utilization values of about 75% orless, the voltage generated by the fuel cell has a roughly linearrelationship with the fuel utilization. At about 75% fuel utilization,the voltage generated can be about 0.7 Volts, with mildly increasingvoltages as the fuel utilization decreases. At fuel utilization valuesof about 80% or greater, the voltage versus utilization curve appears totake on an exponential or power type relationship. From a processstability standpoint, it can be preferable to operate a fuel cell in aportion of the voltage versus utilization curve where the relationshipis linear.

In the other limiting case shown in FIG. 3, the voltage generated by amolten carbonate fuel cell shows a mild increase as the fuel utilizationdecreases. However, in conventional operation, operating a fuel cell atreduced utilization can pose various difficulties. For example, thetotal amount of fuel delivered to a conventionally operated fuel celloperated with lower fuel utilization may need to be reduced, so thatwhatever fuel remains in the anode exhaust/output stream can stillprovide the appropriate amount of heat (upon further combustion) formaintaining the fuel cell temperature. If the fuel utilization isreduced without adjusting the amount of fuel delivered to the fuel cell,the oxidation of the unused fuel may result in higher than desiredtemperatures for the fuel cell. Based at least on these limiting caseconsiderations, conventional fuel cells are typically operated at a fuelutilization of about 70% to about 75%, so as to achieve heat balancewith complete utilization of the fuel.

In contrast to conventional operation, in various embodiments of theinvention, an alternative configuration can be to recycle at least aportion of the exhaust from a fuel cell anode to the input of a fuelcell anode. The output stream from an MCFC anode can include H₂O, CO₂,optionally CO, and optionally but typically unreacted fuel (such as H₂or CH₄) as the primary output components. Instead of using this outputstream as a fuel source to provide heat for a reforming reaction, one ormore separations can be performed on the anode output stream in order toseparate out the CO₂ from the components with potential fuel value, suchas H₂ or CO. The components with fuel value can then be recycled to theinput of an anode.

This type of configuration can provide one or more benefits. First, CO₂can be separated out from the anode output, such as by using a cryogenicCO₂ separator. Several of the components of the anode output (H₂, CO,CH₄) are not easily condensable components, while CO₂ and H₂O can beseparated individually as condensed phases. Depending on the embodiment,at least about 90 vol % of the CO₂ in the anode output can be separatedout to form a relatively high purity CO₂ output stream. Afterseparation, the remaining portion of the anode output can correspondprimarily to components with fuel value, as well as reduced amounts ofCO₂ and/or H₂O. This portion of the anode output after separation can berecycled for use as part of the anode input, along with additional fuel.In this type of configuration, even though the fuel utilization in asingle pass through the MCFC(s) may be low, the unused fuel can beadvantageously recycled for another pass through the anode. As a result,the single-pass fuel utilization can be at a reduced level, whileavoiding loss (exhaust) of unburned fuel to the environment.

Additionally or alternatively to recycling a portion of the anodeexhaust to the anode input, another configuration option can be to use aportion of the anode exhaust as an input for a combustion reaction for aturbine or other combustion power source. The relative amounts of anodeexhaust recycled to the anode input and/or as an input to the combustionzone can be any convenient or desirable amount. If the anode exhaust isrecycled to only one of the anode input and the combustion zone, theamount of recycle can be any convenient amount, such as up to 100% ofthe portion of the anode exhaust remaining after any separation toremove CO₂ and/or H₂O. When a portion of the anode exhaust is recycledto both the anode input and the combustion zone, the total recycledamount by definition can be 100% or less of the remaining portion ofanode exhaust. Otherwise, any convenient split of the anode exhaust canbe used. In various embodiments of the invention, the amount of recycleto the anode input can be at least about 10% of the anode exhaustremaining after separations, for example at least about 25%, at leastabout 40%, at least about 50%, at least about 60%, at least about 75%,or at least about 90%. Additionally or alternately in those embodiments,the amount of recycle to the anode input can be about 90% or less of theanode exhaust remaining after separations, for example about 75% orless, about 60% or less, about 50% or less, about 40% or less, about 25%or less, or about 10% or less. Further additionally or alternately, invarious embodiments of the invention, the amount of recycle to thecombustion zone (turbine) can be at least about 10% of the anode exhaustremaining after separations, for example at least about 25%, at leastabout 40%, at least about 50%, at least about 60%, at least about 75%,or at least about 90%. Additionally or alternately in those embodiments,the amount of recycle to the combustion zone (turbine) can be about 90%or less of the anode exhaust remaining after separations, for exampleabout 75% or less, about 60% or less, about 50% or less, about 40% orless, about 25% or less, or about 10% or less.

Any H₂ present in the anode exhaust can represent a fuel that can becombusted without generating CO₂. Because at least some H₂ can begenerated as part of the anode portion of the fuel cell(s), the CO₂generated during reforming can be primarily removed by the CO₂separation stage(s) in the anode portion of the system. As a result, useof H₂ from the anode exhaust as part of the fuel for the combustionreaction can allow for a situation where the CO₂ generated from“combustion” of the fuel can be created in the anode portion of thesystem, as opposed to having to transport the CO₂ across the membrane.

Recycle of H₂ from the anode exhaust to the combustion reaction canprovide synergistic benefits for a turbine (or other combustion system)that includes an exhaust gas recycle (EGR) configuration. In an EGRconfiguration, a portion of the CO₂-containing exhaust gas from thecombustion reaction can be recycled and used as part of the input gasflow to the turbine. During operation of a combustion-powered turbine,an input gas flow of an oxidant (such as air or oxygen-enriched air) canbe compressed prior to introduction into the combustion reaction. Thecompressors used for the input flows to the combustion reaction can tendto be volume limited, so that a similar number of moles of gas can becompressed, typically regardless of the mass of the gas. However, gaseswith a higher mass can tend to have higher heat capacities and/or canallow for greater pressure ratios across the expander portion of aturbine. A CO₂-enriched exhaust stream can provide a convenient sourceof a gas stream with higher molecular weight components that can allowfor improved conversion of the energy from the combustion reaction intoelectric power from the turbine. Although introducing a CO₂-enrichedstream into the combustion reaction can provide some benefits, there canbe effective limits to the amount of the CO₂-enriched stream that can beadded without significantly (negatively) impacting the combustionreaction. Since the CO₂-enriched stream does not itself typicallycontain “fuel”, the stream can largely act as a diluent within thecombustion reaction. As a result, the amount of recyclable CO₂ can belimited based, at least in part, on maintaining the conditions in thecombustion reaction within an appropriate flammability window.

Recycle of H₂ from the anode exhaust can complement an EGR configurationin one or more ways. First, combustion of H₂ may not directly result ingeneration of CO₂. Instead, as noted above, the CO₂ generated when theH₂ is produced can be generated in the anode loop. This can reduce theamount of CO₂ needing to be transferred from cathode to anode for agiven level of power generation. Additionally, H₂ can also have thebenefit of modifying the operation of the combustion source, such asthrough modifying the flammability window, so that higher concentrationsof CO₂ can be tolerated while still maintaining a desired combustionreaction. Being able to expand the flammability window can allow forincreased concentrations of CO₂ in the combustion exhaust, and thereforeincreased CO₂ in the input to the cathodes of the fuel cell.

The benefit of being able to increase the CO₂ concentration in the inputto the fuel cell cathode can be related to the nature of how a moltencarbonate fuel cell operates. As detailed below, there can be practicallimits in the amount of CO₂ separable by an MCFC from a cathode exhauststream. Depending on the operating conditions, an MCFC can lower the CO₂content of a cathode exhaust stream to about 2.0 vol % or less, e.g.,about 1.5 vol % or less or about 1.2 vol % or less. Due to thislimitation, the net efficiency of CO₂ removal when using moltencarbonate fuel cells can be dependent on the amount of CO₂ in thecathode input. For a combustion reaction using natural gas as a fuel,the amount of CO₂ in the combustion exhaust can correspond to a CO₂concentration at the cathode input of at least about 4 vol %. Use ofexhaust gas recycle can allow the amount of CO₂ at the cathode input tobe increased to at least about 5 vol %, e.g., at least about 6 vol %.Due to the increased flammability window that can be provided when usingH₂ as part of the fuel, the amount of CO₂ added via exhaust gas recyclecan be increased still further, so that concentrations of CO₂ at thecathode input of at least about 7.5 vol % or at least about 8 vol % canbe achieved. Based on a removal limit of about 1.5 vol % at the cathodeexhaust, increasing the CO₂ content at the cathode input from about 5.5vol % to about 7.5 vol % corresponds to a ˜50% increase in the amount ofCO₂ that can be captured using a fuel cell and transported to the anodeloop for eventual CO₂ separation.

The amount of H₂ present in the anode output can be increased, forexample, by using a water gas shift reactor to convert H₂O and COpresent in the anode output into H₂ and CO₂. Water is an expected outputof the reaction occurring at the anode, so the anode output cantypically have an excess of H₂O relative to the amount of CO present inthe anode output. CO can be present in the anode output due toincomplete carbon combustion during reforming and/or due to theequilibrium balancing reactions between H₂O, CO, H₂, and CO₂ (i.e., thewater-gas shift equilibrium) under either reforming conditions or theconditions present during the anode reaction. A water gas shift reactorcan be operated under conditions to drive the equilibrium further in thedirection of forming CO₂ and H₂ at the expense of CO and H₂O. Highertemperatures can tend to favor the formation of CO and H₂O. Thus, oneoption for operating the water gas shift reactor can be to expose theanode output stream to a suitable catalyst, such as a catalyst includingiron oxide, zinc oxide, copper on zinc oxide, or the like, at a suitabletemperature, e.g., between about 190° C. to about 210° C. Optionally,the water-gas shift reactor can include two stages for reducing the COconcentration in an anode output stream, with a first higher temperaturestage operated at a temperature from at least about 300° C. to about375° C. and a second lower temperature stage operated at a temperatureof about 225° C. or less, such as from about 180° C. to about 210° C. Inaddition to increasing the amount of H₂ present in the anode output, thewater-gas shift reaction can also increase the amount of CO₂ at theexpense of CO. This can exchange difficult-to-remove carbon monoxide(CO) for carbon dioxide, which can be more readily removed bycondensation (e.g., cryogenic removal), chemical reaction (such as amineremoval), and/or other CO, removal methods.

In some aspects of the invention, all or substantially all of the anodeoutput stream remaining after separation of (portions of) the CO₂ (andH₂O) can be recycled for use as either an input for the fuel cellanode(s) or as a fuel input for the combustion-powered generator. Thus,for the portion of the anode output stream that remains after awater-gas shift reaction, removal of CO₂, and/or removal of H₂O, atleast about 90% of the remaining content can advantageously be used aseither an input for the fuel cell anode(s) or as a fuel input for thecombustion powered generator. Alternatively, the anode output streamafter separation can be used for more than one purpose, but recycle ofany portion of the anode output stream for use as a direct input to acathode and/or as an input to an oxidizer for heating of the fuel cellcan advantageously be avoided.

FIG. 4 shows an example of the anode flow path portion of agenerator/fuel cell system according to the invention. In FIG. 4, aninitial fuel stream 405 can optionally be reformed 410 to convertmethane (or another type of fuel) and water into H₂ and CO₂.Alternatively, the reforming reaction can be performed in a reformingstage that is part of an assembly including both the reforming stage andthe fuel cell anode 420. Additionally or alternately, at least a portionof fuel stream 405 can correspond to hydrogen gas, so that the amount ofreforming needed to provide fuel to the anode 420 can be reduced and/orminimized. The optionally reformed fuel 415 can then be passed intoanode 420. A recycle stream 455 including fuel components from the anodeexhaust 425 can also serve as an input to the anode 420. A flow ofcarbonate ions 422 from the cathode portion of the fuel cell (not shown)can provide the remaining reactant needed for the anode fuel cellreactions. Based on the reactions in the anode 420, the resulting anodeexhaust 425 can include H₂O, CO₂, one or more components correspondingto unreacted fuel (H₂, CO, CH₄, or others), and optionally one or moreadditional non-reactive components, such as N₂ and/or other contaminantsthat are part of fuel stream 405. The anode exhaust 425 can then bepassed into one or more separation stages 430 for removal of CO₂ (andoptionally also H₂O). A cryogenic CO₂ removal system can be an exampleof a suitable type of separation stage. Optionally, the anode exhaustcan first be passed through a water gas shift reactor 440 to convert anyCO present in the anode exhaust (along with some H₂O) into CO₂ and H₂ inan optionally water gas shifted anode exhaust 445.

An initial portion of the separation stage(s) 430 can be used to removea majority of the H₂O present in the anode exhaust 425 as an H₂O outputstream 432. Additionally or alternately, a heat recovery steam generatorsystem or other heat exchangers independent of the cryogenic separationsystem can be used to remove a portion of the H₂O. A cryogenic CO₂removal system can then remove a majority of the CO₂ as a high purityCO₂ stream 434. A purge stream (not shown) can also be present, ifdesired, to prevent accumulation of inert gases within the anode recycleloop. The remaining components of the anode exhaust stream can then beused either as a recycled input 455 to the inlet of anode 420 or as aninput stream 485 for a combustion powered turbine.

Conventionally, at least some reforming is performed prior to any fuelentering a fuel cell. This initial/preliminary reforming can beperformed in a reformer that is external to the fuel cell(s) or fuelcell stack(s). Alternatively, the assembly for a fuel cell stack caninclude one or more reforming zones located within the stack but priorto the anodes of the fuel cells in the stack. This initial reformingtypically converts at least some fuel into hydrogen prior to enteringthe anode, so that the stream that enters the anode can have sufficienthydrogen to maintain the anode reaction. Without this initial reforming,in certain embodiments, the hydrogen content in the anode can be toolow, resulting in little or no transport of CO₂ from cathode to anode.By contrast, in some embodiments the fuel cell(s) in a fuel cell arraycan be operated without external reforming, i.e., based only onreforming within the anode portion of the fuel cell, due to sufficienthydrogen being present in the recycled portion of the anode exhaust.When a sufficient amount of H₂ is present in the anode feed, such as atleast about 10 vol % of the fuel delivered to the anode in the form ofH₂, the reaction conditions in the anode can allow for additionalreforming to take place within the anode itself, which, depending onflow path, can reduce and/or eliminate the need for a reforming stageexternal (prior) to the anode input(s) in the methods according to theinvention. If the anode feed does not contain a sufficient amount ofhydrogen, the anode reaction can stall, and reforming activity andiorother reactions in the anode can be reduced, minimized, or haltedentirely. As a result, in embodiments where the amount of H₂ present inthe anode feed is insufficient, it may be desirable (or necessary) forthere to be a reforming stage external (prior) to the anode input(s).

Operation of Cathode Portion

In various aspects according to the invention, molten carbonate fuelcells used for carbon capture can be operated to improve or enhance thecarbon capture aspects of the fuel cells, as opposed to (or even at theexpense of) enhancing the power generation capabilities. Conventionally,a molten carbonate fuel cell can be operated based on providing adesirable voltage while consuming all fuel in the fuel stream deliveredto the anode. This can be conventionally achieved in part by using theanode exhaust as at least a part of the cathode input stream. Bycontrast, the present invention uses separate/different sources for theanode input and cathode input. By removing any direct link between thecomposition of the anode input flow and the cathode input flow,additional options become available for operating the fuel cell toimprove capture of carbon dioxide.

One initial challenge in using molten carbonate fuel cells for carbondioxide removal can be that the fuel cells have limited ability toremove carbon dioxide from relatively dilute cathode feeds. FIG. 5 showsan example of the relationship between 1) voltage and CO₂ concentrationand 2) power and CO₂ concentration, based on the concentration of CO₂ inthe cathode input gas. As shown in FIG. 5, the voltage and/or powergenerated by a carbonate fuel cell can start to drop rapidly as the CO₂concentration falls below about 2.0 vol %. As the CO₂ concentrationdrops further, e.g., to below 1.0 vol %, at some point the voltageacross the fuel cell can become low enough that little or no furthertransport of carbonate may occur. Thus, at least some CO₂ is likely tobe present in the exhaust gas from the cathode stage of a fuel cell,pretty much regardless of the operating conditions.

One modification of the fuel cell operating conditions can be to operatethe fuel cell with an excess of available reactants at the anode, suchas by operating with low fuel utilization at the anode, as describedabove. By providing an excess of the reactants for the anode reaction inthe fuel cell, the availability of CO₂ for the cathode reaction can beused as a/the rate limiting variable for the reaction.

When operating MCFCs to enhance the amount of carbon capture, thefactors for balancing can be different than when attempting to improvefuel utilization. In particular, the amount of carbon dioxide deliveredto the fuel cells can be determined based on the output flow from thecombustion generator providing the CO₂-containing stream. To a firstapproximation, the CO₂ content of the output flow from a combustiongenerator can be a minor portion of the flow. Even for a higher CO₂content exhaust flow, such as the output from a coal-fired combustiongenerator, the CO₂ content from most commercial coal fired power plantscan be about 15 vol % or less. In order to perform the cathode reaction,this could potentially include between about 5% and about 15%, typicallybetween about 7% and about 9%, of oxygen used to react with the CO₂ toform carbonate ions. As a result, less than about 25 vol % of the inputstream to the cathode can typically be consumed by the cathodereactions. The remaining at least about 75% portion of the cathode flowcan be comprised of inert/non-reactive species such as N₂, H₂O, andother typical oxidant (air) components, along with any unreacted CO₂ andO₂.

Based on the nature of the input flow to the cathode relative to thecathode reactions, the portion of the cathode input consumed and removedat the cathode can be about 25 vol % or less, for example about 10 vol %or less for input flows based on combustion of cleaner fuel sources,such as natural gas sources. The exact amount can vary based on the fuelused, the diluent content in the input fuel (e.g., N₂ is typicallypresent in natural gas at a small percentage), and the oxidant (air) tofuel ratio at which the combustor is operated, all of which can vary,but are typically well known for commercial operations. As a result, thetotal gas flow into the cathode portions of the fuel cells can berelatively predictable (constant) across the total array of fuel cellsused for carbon capture. Several possible configurations can be used inorder to provide an array of fuel cells to enhance/improve/optimizecarbon capture. The following configuration options can be used alone orin combination as part of the strategy for improving carbon capture.

A typical configuration option can be to divide the CO₂-containingstream between a plurality of fuel cells. The CO₂-containing outputstream from an industrial generator can typically correspond to a largeflow volume relative to desirable operating conditions for a single MCFCof reasonable size. Instead of processing the entire flow in a singleMCFC, the flow can be divided amongst a plurality of MCFC units, usuallyat least some of which are in parallel, so that the flow rate in eachunit can be within a desired flow range.

Additionally or alternately, fuel cells can be utilized in series tosuccessively remove CO₂ from a flow stream. Regardless of the number ofinitial fuel cells to which a CO₂-containing stream can be distributedto in parallel, each initial fuel cell can be followed by one or moreadditional cells in series to further remove additional CO₂. Similar tothe situation demonstrated in FIG. 3 for the H₂ input to the anode,attempting to remove CO₂ within a stream in a single fuel cell couldlead to a low and/or unpredictable voltage output. Rather thanattempting to remove CO₂ to a desired level in a single fuel cell, CO₂can be removed in successive cells until a desired level can beachieved. For example, each cell in a series of fuel cells can be usedto remove some percentage (e.g., about 50%) of the CO₂ present in a fuelstream. In such an example, if three fuel cells are used in series, theCO concentration can be reduced (e.g., to about 15% or less of theoriginal amount present, which can correspond to reducing the CO₂concentration from about 6% to about 1% or less over the course of threefuel cells in series).

Further additionally or alternately, the operating conditions can beselected in early fuel stages in series to provide a desired outputvoltage while the array of stages can be selected to achieve a desiredlevel of carbon capture. As an example, an array of fuel cells can beused with three fuel cells in series. The first two fuel cells in seriescan be used to remove CO₂ while maintaining a desired output voltage.The final fuel cell can then be operated to remove CO₂ to a desiredconcentration.

Still further additionally or alternately, there can be separateconnectivity for the anodes and cathodes in a fuel cell array. Forexample, if the fuel cell array includes fuel cathodes connected inseries, the corresponding anodes can be connected in any convenientmanner, not necessarily matching up with the same arrangement as theircorresponding cathodes, for example. This can include, for instance,connecting the anodes in parallel, so that each anode receives the sametype of fuel feed, and/or connecting the anodes in a reverse series, sothat the highest fuel concentration in the anodes can correspond tothose cathodes having the lowest CO₂ concentration.

Exhaust Gas Recycle

Aside from providing exhaust gas to a fuel cell array for capture andeventual separation of the CO₂, an additional or alternate potential usefor exhaust gas can include recycle back to the combustion reaction toincrease the CO₂ content. Increasing the CO₂ content of a combustionreaction for a gas powered turbine can be used to increase the poweroutput of the turbine. When hydrogen is available for addition to thecombustion reaction, such as hydrogen from the anode exhaust of the fuelcell array, further benefits can be gained from using recycled exhaustgas to increase the CO₂ content within the combustion reaction.

In various aspects of the invention, the exhaust gas recycle loop of apower generation system can receive a first portion of the exhaust gasfrom combustion, while the fuel cell array can receive a second portion.The amount of exhaust gas from combustion recycled to the combustionzone of the power generation system can be any convenient amount, suchas at least about 15% (by volume), for example at least about 25%, atleast about 35%, at least about 45%, or at least about 50%. Additionallyor alternately, the amount of combustion exhaust gas recirculated backto the combustion zone can be about 65% (by volume) or less, e.g., about60% or less, about 55% or less, about 50% or less, or about 45% or less.

In one or more aspects of the invention, a mixture of an oxidant (suchas air and/or oxygen-enriched air) and fuel can be combusted and(simultaneously) mixed with a stream of recycled exhaust gas. The streamof recycled exhaust gas, which can generally include products ofcombustion such as CO₂, can be used as a diluent to control, adjust, orotherwise moderate the temperature of combustion and of the exhaust thatcan enter the succeeding expander. As a result of using oxygen-enrichedair, the recycled exhaust gas can have an increased CO₂ content, therebyallowing the expander to operate at even higher expansion ratios for thesame inlet and discharge temperatures, thereby enabling significantlyincreased power production.

A gas turbine system can represent one example of a power generationsystem where recycled exhaust gas can be used to enhance the performanceof the system. The gas turbine system can have a first/main compressorcoupled to an expander via a shaft. The shaft can be any mechanical,electrical, or other power coupling, thereby allowing a portion of themechanical energy generated by the expander to drive the maincompressor. The gas turbine system can also include a combustion chamberconfigured to combust a mixture of a fuel and an oxidant. In variousaspects of the invention, the fuel can include any suitable hydrocarbongas/liquid, such as natural gas, methane, ethane, naphtha, butane,propane, syngas, diesel, kerosene, aviation fuel, coal derived fuel,bio-fuel, oxygenated hydrocarbon feedstock, or any combinations thereof.The oxidant can, in some embodiments, be derived from a second or inletcompressor fluidly coupled to the combustion chamber and adapted tocompress a feed oxidant. In one or more embodiments of the invention,the feed oxidant can include atmospheric air and/or enriched air. Whenthe oxidant includes enriched air alone or a mixture of atmospheric airand enriched air, the enriched air can be compressed by the inletcompressor (in the mixture, either before or after being mixed with theatmospheric air). The enriched air and/or the air-enriched air mixturecan have an overall oxygen concentration of at least about 25%, e.g., atleast about 30 wt %, at least about 35 wt %, at least about 40 wt % atleast about 45 wt %, or at least about 50 wt %.

The enriched air can be derived from any one or more of several sources.For example, the enriched air can be derived from such separationtechnologies as membrane separation, pressure swing adsorption,temperature swing adsorption, nitrogen plant-byproduct streams, and/orcombinations thereof. The enriched air can additionally or alternatelybe derived from an air separation unit (ASU), such as a cryogenic ASU,for producing nitrogen for pressure maintenance or other purposes. Incertain embodiments of the invention, the reject stream from such an ASUcan be rich in oxygen, having an overall oxygen content from about 50 wt% to about 70 wt %, can be used as at least a portion of the enrichedair and subsequently diluted, if needed, with unprocessed atmosphericair to obtain the desired oxygen concentration.

In addition to the fuel and oxidant, the combustion chamber canoptionally also receive a compressed recycle exhaust gas, such as anexhaust gas recirculation primarily having CO₂ and nitrogen components.The compressed recycle exhaust gas can be derived from the maincompressor, for instance, and adapted to help facilitate combustion ofthe oxidant and fuel, e.g. by moderating the temperature of thecombustion products. As can be appreciated, recirculating the exhaustgas can serve to increase CO₂ concentration.

An exhaust gas directed to the inlet of the expander can be generated asa product of combustion reaction. The exhaust gas can have a heightenedCO₂ content based, at least in part, on the introduction of recycledexhaust gas into the combustion reaction. As the exhaust gas expandsthrough the expander, it can generate mechanical power to drive the maincompressor, to drive an electrical generator, and/or to power otherfacilities.

The power generation system can, in many embodiments, also include anexhaust gas recirculation (EGR) system. In one or more aspects of theinvention, the EGR system can include a heat recovery steam generator(HRSG) and/or another similar device fluidly coupled to a steam gasturbine. In at least one embodiment, the combination of the HRSG and thesteam gas turbine can be characterized as a power-producing closedRankine cycle. In combination with the gas turbine system, the HRSG andthe steam gas turbine can form part of a combined-cycle power generatingplant, such as a natural gas combined-cycle (NGCC) plant. The gaseousexhaust can be introduced to the HRSG in order to generate steam and acooled exhaust gas. The HRSG can include various units for separatingand/or condensing water out of the exhaust stream, transferring heat toform steam, and/or modifying the pressure of streams to a desired level.In certain embodiments, the steam can be sent to the steam gas turbineto generate additional electrical power.

After passing through the HRSG and optional removal of at least someH₂O, the CO₂-containing exhaust stream can, in some embodiments, berecycled for use as an input to the combustion reaction. As noted above,the exhaust stream can be compressed (or decompressed) to match thedesired reaction pressure within the vessel for the combustion reaction.

Example of Integrated System

FIG. 6 schematically shows an example of an integrated system includingintroduction of both CO₂-containing recycled exhaust gas and H₂ from thefuel cell anode exhaust into the combustion reaction for powering aturbine. In FIG. 6, the turbine can include a compressor 102, a shaft104, an expander 106, and a combustion zone 115. An oxygen source 111(such as air and/or oxygen-enriched air) can be combined with recycledexhaust gas 198 and compressed in compressor 102 prior to enteringcombustion zone 115. A fuel 112, such as CH₄, and a stream containing H₂187 can be delivered to the combustion zone. The fuel and oxidant can bereacted in zone 115 and optionally but preferably passed throughexpander 106 to generate electric power. The exhaust gas from expander106 can be used to form two streams, e.g., a CO₂-containing stream 122(that can be used as an input feed for fuel cell array 125) and anotherCO₂-containing stream 192 (that can be used as the input for a heatrecovery and steam generator system 190, which can, for example, enableadditional electricity to be generated using steam turbines 194). Afterpassing through heat recovery system 190, including optional removal ofa portion of H₂O from the CO₂-containing stream, the output stream 198can be recycled for compression in compressor 102. The proportion of theexhaust from expander 106 used for CO₂-containing stream 192 can bedetermined based on the desired amount of CO₂ for addition to combustionzone 115.

The CO₂-containing stream 122 can be passed into a cathode portion (notshown) of a molten carbonate fuel cell array 125. Based on the reactionswithin fuel cell array 125. CO₂ can be separated from stream 122 andtransported to the anode portion (not shown) of the fuel cell array 125.This can result in a cathode output stream 124 depleted in CO₂. Thecathode output stream 124 can then be passed into a heat recovery (andoptional steam generator) system 150 for generation of heat exchangeand/or additional generation of electricity using steam turbines 154.After passing through heat recovery and steam generator system 150, theresulting flue gas stream 156 can be exhausted to the environment and/orpassed through another type of carbon capture technology, such as anamine scrubber.

After transport of CO₂ from the cathode side to the anode side of fuelcell array 125, the anode output 135 can optionally be passed into awater gas shift reactor 170. Water gas shift reactor 170 can be used togenerate additional H₂ and CO₂ at the expense of CO (and H₂O) present inthe anode output 135. The output from the optional water gas shiftreactor 170 can then be passed into one or more separation stages 140,such as a cold box or a cryogenic separator. This can allow forseparation of an H₂O stream 147 and CO₂ stream 149 from the remainingportion of the anode output. The remaining portion of the anode output185 can include unreacted H₂ generated by reforming but not consumed infuel cell array 125. A first portion 145 of the H₂-containing stream 185can be recycled to the input for the anode(s) in fuel cell array 125. Asecond portion 187 of stream 185 can be used as an input for combustionzone 115. A third portion (not shown) can be vented to the atmosphere,used as is for another purpose, and/or treated for subsequent furtheruse. Although FIG. 6 and the description herein schematically details upto three portions, it is contemplated that only one of these threeportions can be exploited, only two can be exploited, or all three canbe exploited according to the invention.

Examples of Operating Ranges

In various embodiments of the invention, the process can be approachedas starting with a combustion reaction for powering a turbine, aninternal combustion engine, or another system where heat and/or pressuregenerated by a combustion reaction can be converted into another form ofpower. The fuel for the combustion reaction can comprise or be hydrogen,a hydrocarbon, and/or any other compound containing carbon that can beoxidized (combusted) to release energy. Except for when the fuelcontains only hydrogen, the composition of the exhaust gas from thecombustion reaction can have a range of CO₂ contents, depending on thenature of the reaction (e.g., from at least about 2 vol % to about 25vol % or less). Thus, in certain embodiments where the fuel iscarbonaceous, the CO₂ content of the exhaust gas can be at least about 2vol %, for example at least about 4 vol %, at least about 5 vol %, atleast about 6 vol %, at least about 8 vol %, or at least about 10 vol %.Additionally or alternately in such carbonaceous fuel embodiments, theCO₂ content can be about 25 vol % or less, for example about 20 vol % orless, about 15 vol % or less, about 10 vol % or less, about 7 vol % orless, or about 5 vol % or less. Exhaust gases with lower relative CO₂contents (for carbonaceous fuels) can correspond to exhaust gases fromcombustion reactions on fuels such as natural gas. Higher relative CO₂content exhaust gases (for carbonaceous fuels) can correspond tooptimized natural gas combustion reactions, such as those with exhaustgas recycle, or combustion reactions on fuels such as coal.

In some aspects of the invention, the fuel for the combustion reactioncan contain at least about 90 wt % of compounds containing five carbonsor less, e.g., at least about 95 wt %. In such aspects, the CO₂ contentof the exhaust gas can be at least about 4 vol %, for example at leastabout 5 vol %, at least about 6 vol %, at least about 7 vol %, or atleast about 7.5 vol %. Additionally or alternately, the CO₂ content ofthe exhaust gas can be about 13 vol % or less. e.g., about 12 vol % orless, about 10 vol % or less, about 9 vol % or less, about 8 vol % orless, about 7 vol % or less, or about 6 vol % or less. The CO₂ contentof the exhaust gas can represent a range of values depending on theconfiguration of the combustion powered generator. Recycle of an exhaustgas can be beneficial for achieving a CO, content of at least about 6vol %, while addition of hydrogen to the combustion reaction can allowfor further increases in CO₂ content to achieve a CO₂ content of atleast about 7.5 vol %.

Other components of the exhaust gas can correspond to any excess oxidant(O₂) from the combustion reaction, water vapor, any incompletecombustion products from carbonaceous material (such as CO), and/orother spectator species present in the fuel source or oxidant source.For example, if air is used as part of the oxidant source, the exhaustgas can include typical components of air such as N₂, H₂O, and othercompounds in minor amounts that are present in air. Depending on thenature of the fuel source, additional species present after combustionbased on the fuel source may include H₂O, H₂S, and other compoundseither present in the fuel and/or that are partial or completecombustion products of compounds present in the fuel. The amount of O₂present in the exhaust can advantageously be sufficient to provide theoxygen needed for the cathode reaction in the fuel cell. Thus, thevolume percentage of O₂ can advantageously be at least 0.5 times theamount of CO₂ in the exhaust. Optionally, as necessary, additional aircan be added to the exhaust to provide sufficient oxidant for thecathode reaction. When some form of air is used as the oxidant, theamount of N₂ in the exhaust can be at least about 50 vol %, e.g., atleast about 60 vol %.

The input gas to the cathode can be similar in composition to theexhaust gas from the combustion reaction. Optionally, if the combustionreaction is performed under stoichiometric or nearly stoichiometricconditions, the exhaust gas may contain insufficient oxygen for thecathode reaction. In this situation, additional oxidant (air) may beadded to either the exhaust gas or to the cathode input. The temperatureand pressure of the exhaust gas from the combustion reaction may besimilar or may differ from the input conditions for the fuel cellcathode. A suitable temperature for operation of an MCFC can be betweenabout 500° C. and about 700° C., e.g. with an inlet temperature of about550° C. and an outlet temperature of about 600° C. By contrast, theoutlet temperature from the combustion reaction and/or correspondingturbine can be significantly higher. Prior to entering the cathode, heatcan be removed from the combustion exhaust, if desired, e.g., to provideheat for other processes, such as reforming the fuel input for theanode.

The cathode of a fuel cell can correspond to a plurality of cathodesfrom an array of fuel cells, as previously described. For the cathodeoutput from the final cathode(s) in an array sequence (typically atleast including a series arrangement, or else the final cathode(s) andthe initial cathode(s) would be the same), the output composition caninclude about 2.0 vol % or less of CO₂ (e.g., about 1.5 vol % or less orabout 1.2 vol % or less). This relatively low concentration can reflectthe loss of CO₂ as carbonate ions transported across the electrolyte inthe fuel cell(s) to the corresponding anode(s). The amount of O₂ in thecathode output can also be reduced, typically in an amount proportionalto the amount of CO₂ removed, which can result in small correspondingincreases in the amount(s) of the other (spectator) species at thecathode exit.

At least three input source components can be used for the anodereaction in the fuel cell. One input source is a fuel source, such as astream containing H₂ and/or a fuel that can be reformed into H₂ (such asmethane or another compound containing carbon and hydrogen). A secondinput source is a recycle feed from the anode output. A third “input”represents the carbonate ions transported across the electrolyte fromthe cathode.

The fuel source input can have a ratio of water to fuel appropriate forreforming the hydrocarbon (or hydrocarbon-like) compound in the fuelsource used to generate hydrogen. For example, if methane is the inputfor reforming to generate H₂, the ratio of water to fuel can be abouttwo to one. To the degree that H₂ is a portion of the fuel, noadditional water may typically be needed in the fuel. The fuel sourcecan also optionally contain (small amounts of) components incidental tothe fuel source (e.g., a natural gas feed can contain some content ofCO₂ as an additional component). For example, a natural gas feed cancontain CO₂, N₂, and/or other inert (noble) gases as additionalcomponents.

For the anode output from the final anode(s) in an array sequence(typically at least including a series arrangement, or else the finalanode(s) and the initial anode(s) would be the same), the outputcomposition from the final anode(s) can include H₂O, CO₂, H₂, optionallyCO, and optionally but typically a smaller portion of unreacted fuel(e.g., CH₄). The anode output can include at least about 25 vol % H₂Oand from about 20 vol % to about 35 vol % CO₂. When the anode isoperated to have a reduced fuel utilization, the amount of H₂ in theanode output can additionally or alternately be from about 10 vol % H₂to about 50 vol % H₂. At the anode output, when present, the amount ofCO can be from about 1 vol % or less to about 10 vol %. Optionally, areforming stage can be included after the anode output to convert CO andH₂O in the anode output into CO₂ and H₂, if desired. The anode outputcan further additionally or alternately include 2 vol % or less ofvarious other components, such as N₂, CH₄ (or other unreactedcarbon-containing fuels), and/or other components.

After passing through the optional reforming stage, the anode output canbe passed through one or more separation stages for removal of waterand/or CO₂ from the anode output stream. A cryogenic CO₂ separator canbe an example of a suitable separator. As the anode output is cooled,the majority of the water in the anode output can be separated out as acondensed (liquid) phase. Further cooling and/or pressurizing of thewater-depleted anode output flow can then separate out high purity CO₂,as the other remaining components in the anode output flow (such as H₂,N₂, CH₄) do not tend to readily form condensed phases. A cryogenic CO₂separator can recover between about 90% and about 99% of the CO₂ presentin a flow, depending on the operating conditions.

Because both water and CO₂ can be readily condensed out from the anodeoutput flow, the stream leaving the separation stage(s) can include fromabout 30 vol % to about 70 vol % H₂, along with 15 vol % or less each ofCO₂, H₂O, CH₄, and/or other components that can be considered spectatorspecies during the anode reaction(s).

Applications for CO₂ Output after Capture

In various aspects of the invention, the systems and methods describedabove can allow for production of carbon dioxide as a pressurizedliquid. For example, the CO₂ generated from a cryogenic separation stagecan initially correspond to a pressurized CO₂ liquid with a purity of atleast about 90%, e.g., at least about 95%, at least about 97%, at leastabout 98%, or at least about 99%. This pressurized CO₂ stream can beused, e.g., for injection into wells in order to further enhance oil orgas recovery such as in secondary oil recovery. When done in proximityto a facility that encompasses a gas turbine, the overall system maybenefit from additional synergies in use of electrical/mechanical powerand/or through heat integration with the overall system.

Alternatively, for systems dedicated to an enhanced oil recovery (EOR)application (i.e., not comingled in a pipeline system with tightcompositional standards), the CO₂ separation requirements may besubstantially relaxed. The EOR application can be sensitive to thepresence of O₂, so O₂ can be absent, in some embodiments, from a CO₂stream intended for use in EOR. However, the EOR application can tend tohave a low sensitivity to dissolved CO, H₂, and/or CH₄. Those dissolvedgases can typically have only subtle impacts on the solubilizing abilityof CO₂ used for EOR. Injecting gases such as CO, H₂, and/or CH₄ as EORgases can result in some loss of fuel value recovery, but such gases canbe otherwise compatible with EOR applications.

Additionally or alternately, a potential use for CO₂ as a pressurizedliquid can be as a nutrient in biological processes such as algaegrowth/harvesting. The use of MCFCs for CO₂ separation can ensure thatmost biologically significant pollutants could be reduced to acceptablylow levels, resulting in a CO₂-containing stream having only minoramounts of other “contaminant” gases (such as CO, H₂, N₂, and the like,and combinations thereof) that are unlikely to substantially negativelyaffect the growth of photosynthetic organisms. This can be in starkcontrast to the output streams generated by most industrial sources,which can often contain potentially highly toxic material such as heavymetals.

In this type of aspect of the invention, the CO₂ stream generated byseparation of CO₂ in the anode loop can be used to produce biofuelsand/or chemicals, as well as precursors thereof. Further additionally oralternately, CO₂ may be produced as a liquid, allowing for much easierpumping and transport across distances, e.g., to large fields ofphotosynthetic organisms. Conventional emission sources can emit hot gascontaining modest amounts of CO₂ (e.g., about 4-15%) mixed with othergases and pollutants. These materials would normally need to be pumpedas a dilute gas to an algae pond or biofuel “farm”. By contrast, theMCFC system according to the invention can produce a concentrated CO₂stream (˜60-70% by volume on a dry basis) that can be concentratedfurther to 95%+ (for example 96%+, 97%+, 98%+, or 99%+) and easilyliquefied. This stream can then be transported easily and efficientlyover long distances at relatively low cost and effectively distributedover a wide area. In these embodiments, residual heat from thecombustion source/MCFC may be integrated into the overall system aswell.

An alternative embodiment may apply where the CO₂ source/MCFC andbiological/chemical production sites are co-located. In that case, onlyminimal compression may be necessary (i.e., to provide enough CO₂pressure to use in the biological production, e.g., from about 15 psigto about 150 psig). Several novel arrangements can be possible in such acase. Secondary reforming may optionally be applied to the anode exhaustto reduce CH₄ content, and water-gas shift may optionally additionallyor alternately be present to drive any remaining CO into CO₂ and H₂.

Additional Embodiments Embodiment 1

A method for capturing carbon dioxide from a combustion source, saidmethod comprising: introducing one or more fuel streams and anO₂-containing stream into a combustion zone; performing a combustionreaction in the combustion zone to generate a combustion exhaust, thecombustion exhaust comprising CO₂; processing at least a first portionof the combustion exhaust with a fuel cell array of one or more moltencarbonate fuel cells to form a cathode exhaust stream from at least onecathode outlet of the fuel cell array, the one or more fuel cells eachhaving an anode and a cathode, the molten carbonate fuel cells beingoperatively connected to the combustion exhaust through one or morecathode inlets of fuel cells in the fuel cell array; reacting carbonatefrom the one or more fuel cell cathodes with hydrogen within the one ormore fuel cell anodes to produce electricity, an anode exhaust streamfrom at least one anode outlet of the fuel cell array comprising CO₂ andH₂; separating CO₂ from the anode exhaust stream in one or moreseparation stages to form a CO₂-depleted anode exhaust stream; andpassing at least a first portion of the CO₂-depleted anode exhauststream to the combustion zone.

Embodiment 2

The method of embodiment 1, further comprising recycling at least asecond portion of the CO₂-depleted anode exhaust stream to the one ormore fuel cell anodes and/or further comprising passingcarbon-containing fuel (e.g., comprising CH₄) into the one or more fuelcell anodes, optionally without passing the carbon-containing fuel intoa reforming stage prior to entering the one or more fuel cell anodes.

Embodiment 3

The method of embodiment 1 or embodiment 2, further comprising:reforming the carbon-containing fuel to generate hydrogen: and passingat least a portion of the generated hydrogen into the one or more fuelcell anodes.

Embodiment 4

The method of any one of the previous embodiments, wherein thecombustion exhaust comprises about 10 vol % or less of CO₂, such asabout 8 vol % or less of CO₂, the combustion exhaust optionallycomprising at least about 4 vol % of CO₂.

Embodiment 5

The method of any one of the previous embodiments, further comprisingrecycling a CO₂-containing stream from the combustion exhaust (e.g.,which can comprise at least about 6 vol % CO₂) to the combustion zone,which recycling can optionally comprise: exchanging heat between asecond portion of the combustion exhaust and an H₂O-containing stream toform steam; separating water from the second portion of the combustionexhaust to form an H₂O-depleted combustion exhaust stream; and passingat least a portion of the H₂O-depleted combustion exhaust stream intothe combustion zone.

Embodiment 6

The method of any one of the previous embodiments, wherein the anodeexhaust stream comprises at least about 5.0 vol % of H₂, such as atleast about 10 vol % or at least about 15 vol %.

Embodiment 7

The method of any one of the previous embodiments, further comprisingexposing the anode exhaust stream to a water gas shift catalyst, ahydrogen content of the anode exhaust stream after exposure to the watergas shift being greater than a hydrogen content of the anode exhauststream prior to the exposure.

Embodiment 8

The method of any one of the previous embodiments, wherein a fuelutilization of fuel cell anodes in the fuel cell array is about 45% toabout 65%, such as about 60% or less.

Embodiment 9

The method of any one of the previous embodiments, wherein the firstportion of the CO₂-depleted anode exhaust stream is combined with a fuelstream prior to passing the first portion of the CO₂-depleted anodeexhaust stream into the combustion zone.

Embodiment 10

The method of any one of the previous embodiments, wherein a cathodeexhaust stream has a CO₂ content of about 2.0 vol % or less, such as 1.5vol % or less or about 1.2 vol % or less.

Embodiment 11

The method of any one of the previous embodiments, wherein separatingCO₂ from the anode exhaust stream comprises cooling the anode exhauststream to form a condensed phase of CO₂, and optionally furthercomprising separating water from the anode exhaust stream prior toforming the condensed phase of CO₂.

Embodiment 12

A system for power generation comprising: a combustion turbine includinga compressor, the compressor receiving an oxidant input and being influid communication with a combustion zone, the combustion zone furtherreceiving a first fuel input and a second fuel input, the combustionzone being in fluid communication with an expander having an exhaustoutput; an exhaust gas recirculation system providing fluidcommunication between a first portion of the expander exhaust output andthe combustion zone, e.g., by passing the first portion of the expanderexhaust output into the compressor, which exhaust gas recirculationsystem optionally further comprises a heat recovery steam generationsystem; a fuel cell array having at least one cathode input, at leastone cathode output, at least one anode input, and at least one anodeoutput, a second portion of the expander exhaust output being in fluidcommunication with the at least one cathode input; and an anode recycleloop comprising one or more carbon dioxide separation stages, a firstportion of an anode recycle loop output being provided to the combustionzone as at least a portion of the second fuel input.

Embodiment 13

The system of embodiment 12, wherein a second portion of the anoderecycle loop output is provided to the anode input.

Embodiment 14

The system of embodiment 12 or embodiment 13, wherein the anode recycleloop further comprises a water gas shift reaction zone, the anode inputpassing through the water gas shift reaction zone prior to at least onestage of the one or more carbon dioxide separation stages.

Embodiment 15

The system of any one of embodiments 12-14, wherein the first fuel inputand the second fuel input are combined prior to entering the combustionzone.

EXAMPLES

A series of simulations were performed in order to demonstrate thebenefits of using an improved configuration for using a fuel cell forCO₂ separation. The simulations were based on empirical models for thevarious components in the power generation system. The simulations werebased on determining steady state conditions within a system based onmass balance and energy balance considerations.

For the combustion reaction for the turbine, the model included anexpected combustion energy value and expected combustion products foreach fuel component in the feed to the combustion zone (such as C₁-C₄hydrocarbon, H₂, and/or CO). This was used to determine the combustionexhaust composition. An initial reforming zone prior to the anode can beoperated using an “idealized” reforming reaction to convert CH₄ to H₂.The anode reaction was modeled to also operate to perform furtherreforming during anode operation. It is noted that the empirical modelfor the anode did not require an initial H₂ concentration in the anodefor the reforming in the anode to take place. Both the anode and cathodereactions were modeled to convert expected inputs to expected outputs ata utilization rate that was selected as a model input. The model for theinitial reforming zone and the anode/cathode reactions included anexpected amount of heat energy needed to perform the reactions. Themodel also determined the electrical current generated based on theamount of reactants consumed in the fuel cell and the utilization ratesfor the reactants based on the Nernst equation. For species that wereinput to either the combustion zone or the anode/cathode fuel cell thatdid not directly participate in a reaction within the modeled component,the species were passed through the modeled zone as part of the exhaustor output.

In addition to the chemical reactions, the components of the system hadexpected heat input/output values and efficiencies. For example, thecryogenic separator had an energy that was required based on the volumeof CO₂ and H₂O separated out, as well as an energy that was requiredbased on the volume of gas that was compressed and that remained in theanode output flow. Expected energy consumption was also determined for awater gas shift reaction zone, if present, and for compression ofrecycled exhaust gas. An expected efficiency for electric generationbased on steam generated from heat exchange was also used in the model.

The basic configuration used for the simulations included a combustionturbine combine including a compressor, a combustion zone, and anexpander. In the base configuration, a natural gas fuel input wasprovided to the combustion zone. The natural gas input included ˜93%CH₄, ˜2% C₂H₆, ˜2% CO₂, and ˜3% N₂. The oxidant feed to the compressorhad a composition representative of air, including about 70% N₂ andabout 18% O₂. After passing through the expander, a portion of thecombustion exhaust gas was passed through a heat recovery steamgeneration system and then recycled to the compressor. The remainder ofthe combustion exhaust was passed into the fuel cell cathode. Afterpassing through the fuel cell cathode, the cathode exhaust exited thesystem. Unless otherwise specified, the portion of the combustionexhaust recycled back to the combustion zone was ˜35%. This recycledportion of the combustion exhaust served to increase the CO₂ content ofthe output from the combustion zone. Because the fuel cell area wasselected to reduce the CO₂ concentration in the cathode output to afixed value of −1.45%, recycling the combustion exhaust was found toimprove the CO₂ capture efficiency.

In the base configuration, the fuel cell was modeled as a single fuelcell of an appropriate size to process the combustion exhaust. This wasdone to represent use of a corresponding plurality of fuel cells (fuelcell stacks) arranged in parallel having the same active area as themodeled cell. Unless otherwise specified, the fuel utilization in theanode of the fuel cell was set to ˜75%. The fuel cell area was allowedto vary, so that the selected fuel utilization results in the fuel celloperating at a constant fuel cell voltage of ˜0.7 volts and a constantCO₂ cathode output/exhaust concentration of ˜1.45 vol %.

In the base configuration, an anode fuel input flow provided the naturalgas composition described above as a feed to the anode. Steam was alsopresent to provide a steam to carbon ratio in the input fuel of ˜2:1.Optionally, the natural gas input can undergo reforming to convert aportion of the CH₄ in the natural gas to H₂ prior to entering the anode.When a prior reforming stage is present, ˜20% of the CH₄ could bereformed to generate H₂ prior to entering the anode. The anode outputwas passed through a cryogenic separator for removal of H₂O and CO₂. Theremaining portion of the anode output after separation was processeddepending on the configuration for each Example.

For a given configuration, a variety of values could be calculated atsteady state. For the fuel cell, the amount of CO₂ in the anode exhaustand the amount of O₂ in the cathode exhaust was determined. The voltagefor the fuel cell was fixed at ˜0.7 V within each calculation. Forconditions that could result in a higher maximum voltage, the voltagewas stepped down in exchange for additional current, in order tofacilitate comparison between simulations. The area of fuel cellrequired to achieve a final cathode exhaust CO₂ concentration of ˜1.45vol % was also determined to allow for determination of a currentdensity per fuel cell area.

Another set of values were related to CO₂ emissions. The percentage ofCO₂ captured by the system was determined based on the total CO₂generated versus the amount of CO₂ (in Mtons/year) captured and removedvia the cryogenic separator. The CO₂ not captured corresponded to CO₂“lost” as part of the cathode exhaust. Based on the amount of CO₂captured, the area of fuel cell required per ton of CO₂ captured couldalso be determined.

Other values determined in the simulation included the amount of H₂ inthe anode feed relative to the amount of carbon and the amount of N₂ inthe anode feed. It is noted that the natural gas used for both thecombustion zone and the anode feed included a portion of N₂, as would beexpected for a typical real natural gas feed. As a result, N₂ waspresent in the anode feed. The amount of heat (or equivalently steam)required for heating the anode feed for reforming was also determined. Asimilar power penalty was determined based on the power required forcompression and separation in the cryogenic separation stages. Forconfigurations where a portion of the anode exhaust was recycled to thecombustion turbine, the percentage of the turbine fuel corresponding toH₂ was also determined. Based on the operation of the turbine, the fuelcell, and the excess steam generated, as well as any power consumed forheating the reforming zone, compression, and/or separation, a total netpower was determined for the system to allow for a net electricalefficiency to be determined based on the amount of natural gas (or otherfuel) used as an input for the turbine and the anode.

FIGS. 7 and 8 show results from simulations performed based on severalconfiguration variants. FIG. 7 shows configurations corresponding to abase configuration as well as several configurations where a portion ofthe anode output was recycled to the anode input. In FIG. 7, a firstconfiguration (1a) was based on passing the remaining anode output afterthe carbon dioxide and water separation stage(s) into a combustorlocated after the turbine combustion zone. This provided heat for thereforming reaction and also provided additional carbon dioxide for thecathode input. Configuration 1a was representative of a conventionalsystem, such as the aforementioned Manzolini reference, with theexception that the Manzolini reference did not describe recycle ofexhaust gas. Use of the anode output as a feed for the combustorresulted in a predicted fuel cell area of ˜208 km² in order to reducethe CO₂ content of the cathode output to ˜1.45 vol %. The amount of CO₂lost as part of the cathode exhaust was ˜111 lbs CO₂/MWhr. Due to thelarge fuel cell area required for capturing the CO₂, the net powergenerated was ˜724 MW per hour. Based on these values, the amount offuel cell area needed to capture a fixed amount of CO₂ could becalculated, such as an area of fuel cell needed to capture a megaton ofCO₂ during a year of operation. For Configuration 1a, the area of fuelcell required was ˜101.4 km²*year/Mton-CO₂. The efficiency forgeneration of electrical power relative to the energy content of allfuel used in the power generation system was ˜58.9%. By comparison, theelectrical efficiency for the turbine without any form of carbon capturewas ˜61.1%.

In a second set of configurations (2a-2e), the anode output was recycledto the anode input. Configuration 2a represented a basic recycle of theanode output after separation to the anode input. Configuration 2bincluded a water gas shift reaction zone prior to the carbon dioxideseparation stages. Configuration 2c did not include a reforming stageprior to the anode input. Configuration 2d included a reforming stage,but was operated with a fuel utilization of ˜50% instead of ˜75%.Configuration 2e was operated with a fuel utilization of ˜50% and didnot have a reforming stage prior to the anode.

Recycling the anode output back to the anode input, as shown inConfiguration 2a, resulted in a reduction of the required fuel cell areato ˜161 km². However, the CO₂ loss from the cathode exhaust wasincreased to ˜123 lbs COZ/MWhr. This was due to the fact that additionalCO₂ was not being added to the cathode input by the combustion of anodeexhaust in a combustor after the turbine. Instead, the CO₂ content ofthe cathode input was based only on the CO₂ output of the combustionzone. The net result in Configuration 2a was a lower area of fuel cellper ton of CO, captured of ˜87.5 km²*year/Mton-CO₂, but a modestlyhigher amount of CO₂ emissions. Due to the reduced fuel cell area, thetotal power generated was ˜661 MW. Although the net power generated inConfiguration 2a was about 10% less than the net power in Configuration1a, the fuel cell area was reduced by more than 20%. The electricalefficiency was ˜58.9%.

In Configuration 2b, the additional water gas shift reaction zoneincreased the hydrogen content delivered to the anode, which reduced theamount of fuel needed for the anode reaction. Including the water gasshift reaction zone in Configuration 2b resulted in a reduction of therequired fuel cell area to ˜152 km². The CO₂ loss from the cathodeexhaust was ˜123 lbs CO₂/MWhr. The area of fuel cell per megaton of CO₂captured was ˜82.4 km²*year/Mton-CO₂. The total power generated was ˜664MW. The electrical efficiency was ˜59.1%.

Configuration 2c can take further advantage of the hydrogen content inthe anode recycle by eliminating the reforming of fuel occurring priorto entering the anode. In Configuration 2c, reforming can still occurwithin the anode itself. However, in contrast to a conventional systemincorporating a separate reforming stage prior to entry into the fuelcell anode, Configuration 2c relied on the hydrogen content of therecycled anode gas to provide the minimum hydrogen content forsustaining the anode reaction. Because a separate reforming stage wasnot required, the heat energy was not consumed to maintain thetemperature of the reforming stage. Configuration 2c resulted in areduction of the required fuel cell area to ˜149 km². The CO₂ loss fromthe cathode exhaust was ˜122 lbs CO₂/MWhr. The area of fuel cell per tonof CO₂ captured was ˜80.8 km²*year/Mton-CO₂. The total power generatedwas ˜676 MW. The electrical efficiency was ˜60.2%. Based on thesimulation results, eliminating the reforming step seemed to have only amodest impact on the required fuel cell area, but the electricalefficiency appeared to be improved by about 1% relative to Configuration2b. For an industrial scale power generation plant, an efficiencyimprovement of even only 1% is believed to represent an enormousadvantage over the course of a year in power generation.

In Configuration 2d, reforming was still performed to convert ˜20% ofthe methane input to the anode into H₂ prior to entering the anode.Instead, the fuel utilization within the anode was reduced from ˜75% to˜50%. This resulted in a substantial reduction of the required fuel cellarea to ˜113 km². The CO₂ loss from the cathode exhaust was ˜123 lbsCO₂/MWhr. The area of fuel cell per ton of CO₂ captured was ˜61.3km²*year/Mton-CO₂. The total power generated was ˜660 MW. The electricalefficiency was ˜58.8%. Based on the simulation results, reducing thefuel utilization provided a substantial reduction in fuel cell area.Additionally, in comparison with Configurations 2b and 2e, Configuration2d unexpectedly provided the lowest fuel cell area for achieving thedesired level of CO₂ removal.

Configuration 2c incorporated both the reduced fuel utilization of ˜50%as well as elimination of the reforming stage prior to the anode inlet.This configuration provided a combination of improved electricalefficiency and reduced fuel cell area. However, the fuel cell area wasslightly larger than the fuel cell area required in Configuration 2d.This was surprising, as eliminating the reforming stage prior to theanode inlet in Configuration 2c reduced the fuel cell area in comparisonwith Configuration 2b. Based on this, it would have been expected thatConfiguration 2e would provide a further reduction in fuel cell arearelative to Configuration 2d. In Configuration 2e, the CO₂ loss from thecathode exhaust was ˜124 lbs CO₂/MWhr. The area of fuel cell per ton ofCO₂ captured of ˜65.0 km²*year/Mton-CO₂. The total power generated was˜672 MW. The electrical efficiency was ˜59.8%. It is noted thatConfiguration 2d generated only 2% less power than Configuration 2e,while the fuel cell area of Configuration 2d was at least 6% lower thanConfiguration 2e.

FIG. 8 shows simulation results for additional configurations thatincluded recycle of at least a portion of the anode exhaust to thecombustion zone for the turbine. In FIG. 8, Configuration 1b was similarto Configuration 1a (shown in FIG. 7), but also included a water-gasshift reaction stage prior to the CO₂ separation stages. Thus,Configuration 1b was representative of a conventional system, such asthe aforementioned Manzolini reference, with the exceptions that theManzolini reference did not describe a water-gas shift reaction stage orrecycle of exhaust gas. The required fuel cell area to achieve a CO₂concentration in the cathode exhaust of ˜1.45% was ˜190 km². The amountof CO₂ lost as part of the cathode exhaust was ˜117 lbs CO₂/MWhr. Thearea of fuel cell per ton of CO₂ captured was ˜97.6 km²*year/Mton-CO₂.The total power generated was ˜702 MW. The electrical efficiency was˜59.1%.

Configurations 3a, 3b, and 3d correspond to configurations where theanode output was used as an input for the combustion zone of theturbine. In these configurations, the H₂ content of the anode output wasavailable for use as a fuel in the turbine combustion zone. Thisappeared to be advantageous, as the carbon-containing fuel used togenerate the H₂ was generated in the anode recycle loop, where themajority of the resulting CO₂ can be removed via the cryogenicseparation stages. This could also result in a reduction of the amountof carbon containing fuel delivered to the combustion zone, but thereduction in carbon-containing fuel in the combustion zone could alsoresult in the reduction of the CO₂ concentration in the input to thecathode.

Configuration 3a was a configuration similar to Configuration 1a, butwith recycle of the anode exhaust to the combustion zone. The requiredfuel cell area to achieve a CO₂ concentration in the cathode exhaust of˜1.45% was ˜186 km². The amount of CO₂ lost as part of the cathodeexhaust was ˜114 lbs CO₂/MWhr. The area of fuel cell per ton of CO₂captured was ˜100.3 km²*year/Mton-CO₂. The total power generated was˜668 MW. The electrical efficiency was ˜59.7%. Relative to Configuration1a, Configuration 3a had a lower total amount of CO₂ generated (˜2.05Mtons/year for Configuration 1a vs.˜1.85 Mtons/year for Configuration3a). This was believed to be due to the reduced amount ofcarbon-containing fuel delivered to the combustion zone. However, thisalso appeared to result in a reduced CO₂ concentration delivered to thecathode input, which caused the model to show a reduced efficiency ofCO₂ removal for Configuration 3a. As a result, the net amount of CO₂exiting in the cathode exhaust was comparable for Configuration 1a andConfiguration 3a. However. Configuration 3a appeared to have severaladvantages relative to Configuration 1a. First, Configuration 3arequired a lower fuel cell area, so that the system in Configuration 3awould likely have a reduced cost. Additionally, the system inConfiguration 3a appeared to have improved electrical efficiency, whichcan indicate lower fuel usage, even after adjusting for the differentpower output of the configurations.

Configuration 3b was similar to Configuration 3a, but also included awater gas shift reaction zone prior to the cryogenic separation stages.The required fuel cell area to achieve a CO₂ concentration in thecathode exhaust of ˜1.45% was ˜173 km². The amount of CO₂ lost as partof the cathode exhaust was ˜124 lbs CO₂/MWhr. The area of fuel cell perton of CO₂ captured was ˜96.1 km²*year/Mton-CO₂. The total powergenerated was ˜658 MW. The electrical efficiency was ˜59.8%.Configuration 3 b appeared to have increased CO₂ emission via thecathode exhaust. This was believed to be due to the additional hydrogendelivered to the combustion zone, which can result in a correspondingreduction in the amount of CO₂ the combustion exhaust used for thecathode input. However, the fuel cell area was further reduced.

Configuration 3d was similar to Configuration 3b, but the anode fuelutilization was reduced from ˜75% to ˜50%. The required fuel cell areato achieve a CO₂ concentration in the cathode exhaust of ˜1.45% was ˜132km². The amount of CO₂ lost as part of the cathode exhaust was ˜128 lbsCO₂/MWhr. The area of fuel cell per ton of CO₂ captured was ˜77.4km²*year/Mton-CO₂. The total power generated was ˜638 MW. The electricalefficiency was ˜60.7%. Based on the simulation results, reducing thefuel utilization in the anode appeared to result in a substantialimprovement in electrical efficiency relative to Configuration 3b. Thiswas believed to be due to the additional hydrogen delivered to thecombustion zone for the turbine. For comparison, the electricalefficiency of the turbine without any carbon capture was ˜61.1%. Thus,the combination of recycling anode exhaust to the combustion zone andlower fuel utilization appeared to allow an electrical efficiency to beachieved approaching the efficiency without a carbon capture system.

Configurations 4d and 4e represent configurations where the remaininganode exhaust after separation (removal) of CO₂ and H₂O was dividedevenly between recycle to the anode input and recycle to the combustionzone for the turbine. In order to provide sufficient hydrogen for boththe anode input and the combustion zone, the anode fuel utilization inConfigurations 4d and 4e was set to ˜50%. Configurations 4d and 4e bothincluded a water gas shift reaction zone prior to the separation stages.Configuration 4d included a separate reforming stage for reforming ˜20%of the additional fuel input to the anode prior to the fuel entering theanode. Configuration 4e did not include a reforming stage prior to thefuel entering the anode input.

Configuration 4d appeared to show the benefits of recycling the anodeexhaust to both the anode input and the combustion zone. Relative toConfiguration 2d, Configuration 4d appeared to provide an electricalefficiency about a full percentage point greater. Relative toConfiguration 3d, Configuration 4d provided a reduced fuel cell area. InConfiguration 4d, the required fuel cell area to achieve a CO₂concentration in the cathode exhaust of ˜1.45% was ˜122 km². The amountof CO₂ lost as part of the cathode exhaust was ˜126 lbs CO₂/MWhr. Thearea of fuel cell per ton of CO₂ captured was ˜63.4 km²*year/Mton-CO₂.The total power generated was ˜650 MW. The electrical efficiency was˜59.9%.

Removing the pre-reforming stage in Configuration 4e appeared to providefurther benefits. The required fuel cell area to achieve a CO₂concentration in the cathode exhaust of ˜1.45% was ˜112 km². The amountof CO₂ lost as part of the cathode exhaust was ˜126 lbs CO₂/MWhr. Thearea of fuel cell per ton of CO₂ captured was ˜63.4 km²*year/Mton-CO₂.The total power generated was ˜665 MW. The electrical efficiency was˜61.4%. It is noted that the electrical efficiency was actually greaterthan the efficiency of the turbine without any type of carbon capture(˜61.1%).

Although the present invention has been described in terms of specificembodiments, it is not so limited. Suitable alterations/modificationsfor operation under specific conditions should be apparent to thoseskilled in the art. It is therefore intended that the following claimsbe interpreted as covering all such alterations/modifications as fallwithin the true spirit/scope of the invention.

What is claimed is:
 1. A method for capturing carbon dioxide from acombustion source, the method comprising: introducing one or more fuelstreams and an O₂-containing stream into a combustion zone; performing acombustion reaction in the combustion zone to generate a combustionexhaust, the combustion exhaust comprising CO₂; processing at least afirst portion of the combustion exhaust with a fuel cell array of one ormore molten carbonate fuel cells to form a cathode exhaust stream fromat least one cathode outlet of the fuel cell array, the one or more fuelcells each having an anode and a cathode, the molten carbonate fuelcells being operatively connected to the combustion exhaust through oneor more cathode inlets of fuel cells in the fuel cell array; reactingcarbonate from the one or more fuel cell cathodes with hydrogen withinthe one or more fuel cell anodes to produce electricity, an anodeexhaust stream from at least one anode outlet of the fuel cell arraycomprising CO₂ and H₂; separating CO₂ from the anode exhaust stream inone or more separation stages to form a CO₂-depleted anode exhauststream; and passing at least a first portion of the CO₂-depleted anodeexhaust stream to the combustion zone.
 2. The method of claim 1, furthercomprising recycling at least a second portion of the CO₂-depleted anodeexhaust stream to the one or more fuel cell anodes.
 3. The method ofclaim 2, further comprising passing carbon-containing fuel into the oneor more fuel cell anodes.
 4. The method of claim 3, further comprising:reforming the carbon-containing fuel to generate hydrogen; and passingat least a portion of the generated hydrogen into the one or more fuelcell anodes.
 5. The method of claim 3, wherein the carbon-containingfuel is passed into the one or more fuel cell anodes without passing thecarbon-containing fuel into a reforming stage prior to entering the oneor more fuel cell anodes.
 6. The method of claim 3, wherein thecarbon-containing fuel comprises CH₄.
 7. The method of claim 1, whereinthe combustion exhaust comprises about 10 vol % or less of CO₂, such asabout 8 vol % or less of CO₂, the combustion exhaust optionallycomprising at least about 4 vol % of CO₂.
 8. The method of claim 1,further comprising recycling a CO₂-containing stream from the combustionexhaust to the combustion zone.
 9. The method of claim 8, whereinrecycling the CO₂-containing stream from the combustion exhaust to thecombustion zone comprises: exchanging heat between a second portion ofthe combustion exhaust and an H₂O-containing stream to form steam;separating water from the second portion of the combustion exhaust toform an H₂O-depleted combustion exhaust stream; and passing at least aportion of the H₂O-depleted combustion exhaust stream into thecombustion zone.
 10. The method of claim 8, wherein the combustionexhaust comprises at least about 6 vol % CO₂.
 11. The method of claim 1,wherein the anode exhaust stream comprises at least about 5.0 vol % ofH₂, such as at least about 10 vol % or at least about 15 vol %.
 12. Themethod of claim 1, further comprising exposing the anode exhaust streamto a water gas shift catalyst, a hydrogen content of the anode exhauststream after exposure to the water gas shift being greater than ahydrogen content of the anode exhaust stream prior to the exposure. 13.The method of claim 1, wherein a fuel utilization of fuel cell anodes inthe fuel cell array is about 45% to about 65%, such as about 60% orless.
 14. The method of claim 1, wherein the first portion of theCO₂-depleted anode exhaust stream is combined with a fuel stream priorto passing the first portion of the CO₂-depleted anode exhaust streaminto the combustion zone.
 15. The method of claim 1, wherein a cathodeexhaust stream has a CO₂ content of about 2.0 vol % or less, such as 1.5vol % or less or about 1.2 vol % or less.
 16. The method of claim 1,wherein separating CO₂ from the anode exhaust stream comprises coolingthe anode exhaust stream to form a condensed phase of CO₂.
 17. Themethod of claim 16, further comprising separating water from the anodeexhaust stream prior to forming the condensed phase of CO₂.
 18. A systemfor power generation, comprising: a combustion turbine including acompressor, the compressor receiving an oxidant input and being in fluidcommunication with a combustion zone, the combustion zone furtherreceiving a first fuel input and a second fuel input, the combustionzone being in fluid communication with an expander having an exhaustoutput; an exhaust gas recirculation system providing fluidcommunication between a first portion of the expander exhaust output andthe combustion zone; a fuel cell array having at least one cathodeinput, at least one cathode output, at least one anode input, and atleast one anode output, a second portion of the expander exhaust outputbeing in fluid communication with the at least one cathode input; and ananode recycle loop comprising one or more carbon dioxide separationstages, a first portion of an anode recycle loop output being providedto the combustion zone as at least a portion of the second fuel input.19. The system of claim 18, wherein a second portion of the anoderecycle loop output is provided to the anode input.
 20. The system ofclaim 18, wherein the anode recycle loop further comprises a water gasshift reaction zone, the anode input passing through the water gas shiftreaction zone prior to at least one stage of the one or more carbondioxide separation stages.
 21. The system of claim 18, the exhaust gasrecirculation system further comprising a heat recovery steam generationsystem.
 22. The system of claim 18, wherein the exhaust gasrecirculation system provides fluid communication between a firstportion of the expander exhaust output and the combustion zone bypassing the first portion of the expander exhaust output into thecompressor.
 23. The system of claim 18, wherein the first fuel input andthe second fuel input are combined prior to entering the combustionzone.
 24. A method for capturing carbon dioxide from a combustionsource, the method comprising: introducing one or more fuel streams andan O₂-containing stream into a reaction zone; performing a combustionreaction in the combustion zone to generate a combustion exhaust, thecombustion exhaust comprising about 10 vol % CO₂ or less; processing atleast a first portion of the combustion exhaust with a fuel cell arrayof one or more molten carbonate fuel cells, the one or more fuel cellseach having an anode and a cathode, the molten carbonate fuel cellsbeing operatively connected to the combustion exhaust through one ormore cathode inlets of fuel cells in the fuel cell array, a cathodeexhaust stream from at least one cathode outlet of the fuel cell arraycontaining about 2.0 vol % or less of carbon dioxide; reacting carbonatefrom the one or more fuel cell cathodes with hydrogen within the one ormore fuel cell anodes to produce electricity, an anode exhaust streamfrom at least one anode outlet of the fuel cell array comprising CO₂ andat least about 5 vol % H₂, at least a portion of the H₂ reacted with thecarbonate comprising H₂ recycled from the anode exhaust stream;separating CO₂ from the anode exhaust stream in one or more separationstages to form a CO₂-depleted anode exhaust stream; passing at least afirst portion of the CO₂-depleted anode exhaust stream to the combustionzone; and recycling at least a second portion of the CO₂-depleted anodeexhaust stream to one or more of the fuel cell anodes.
 25. The method ofclaim 24, further comprising passing carbon-containing fuel into the oneor more fuel cell anodes.
 26. The method of claim 25, wherein thecarbon-containing fuel is passed into the one or more fuel cell anodeswithout passing the carbon-containing fuel into a reforming stage priorto entering the one or more fuel cell anodes.
 27. The method of claim24, further comprising recycling a CO₂-containing stream from thecombustion exhaust to the combustion zone.
 28. The method of claim 27,wherein recycling the CO₂-containing stream from the combustion exhaustto the combustion zone comprises: exchanging heat between a secondportion of the combustion exhaust and an H₂O-containing stream to formsteam; separating water from the second portion of the combustionexhaust to form an H₂O-depleted combustion exhaust stream; and passingat least a portion of the H₂O-depleted combustion exhaust stream intothe combustion zone.
 29. The method of claim 27, wherein the combustionexhaust comprises at least about 6 vol % CO₂.
 30. The method of claim24, further comprising exposing the anode exhaust stream to a water gasshift catalyst, a hydrogen content of the anode exhaust stream afterexposure to the water gas shift being greater than a hydrogen content ofthe anode exhaust stream prior to the exposure.
 31. The method of claim24, wherein a fuel utilization of fuel cell anodes in the fuel cellarray is about 45% to about 65%.
 32. The method of claim 24, wherein thefirst portion of the CO₂-depleted anode exhaust stream is combined witha fuel stream prior to passing the first portion of the CO₂-depletedanode exhaust stream into the combustion zone.
 33. The method of claim24, further comprising: reforming a carbon-containing fuel to generatehydrogen; and passing at least a portion of the generated hydrogen intothe one or more fuel cell anodes.
 34. The method of claim 33, whereinthe carbon-containing fuel comprises CH₄.
 35. The method of claim 24,wherein the cathode exhaust stream has a CO₂ content of about 1.5 vol %or less.
 36. The method of claim 24, wherein separating CO₂ from theanode exhaust stream comprises cooling the anode exhaust stream to forma condensed phase of CO₂.
 37. The method of claim 36, further comprisingseparating water from the anode exhaust stream prior to forming thecondensed phase of CO₂.